National Park Service officials are preparing to work on removing the black scum coating areas of the Jefferson Memorial in Washington. (Nikki Kahn/The Washington Post)

It’s black! It’s creepy! And it’s crawling over the Jefferson Memorial!

It’s biofilm — a microbial invasion of uncertain origin that has begrimed the stone surface of one of the nation’s most hallowed monuments.

Part algae, part bacteria, part fungi, the biofilm won’t eat your flesh, like the gooey Blob in the 1958 horror film, as a National Park Service spokesman remarked.

But it’s not clear if it’s munching on the stone.

And it can’t be killed.

A variety of samples are being tested to see which will work best and prove to be the least harmful as National Park Service officials get ready to work on removing the black scum coating areas of Jefferson Memorial in Washington. (Nikki Kahn/The Washington Post)

It has given the elegant white memorial on Washington’s Tidal Basin a dingy look, and, Blob-like, it is growing.

It is especially pronounced on the memorial’s dome, around its base, and on the triangular pediment that portrays Jefferson and four colleagues who helped draft the Declaration of Independence.

Concerned citizens have offered to try to clean it off.

And the Park Service is experimenting with several cleaning solutions to see what works best without harming the Vermont marble.

No damaging scrub brushes can be used, although some of the cleaning products leave a temporary orange tint. Officials have also discussed cleaning with lasers.

The film is also on the memorial amphitheater at Arlington National Cemetery, the Washington Monument and the Lincoln Memorial, the Park Service said.

A variety of samples are being tested to see which will work best and prove to be the least harmful as National Park Service officials get ready to work on removing the black scum coating areas of Jefferson Memorial in Washington. (Nikki Kahn/The Washington Post)

It was on the D.C. War Memorial on the Mall, the District’s tribute to its World War I dead, before that monument was refurbished in 2011. And it may be coming back there, too.

It’s somewhat akin to other biofilms, like dental plaque, the Park Service said.

But what, exactly, is it?

“We don’t even know the who, what, when, where, why,” said Judy Jacob, senior conservator with the Park Service’s Historic Architecture, Conservation, and Engineering Center in New York.

“We’re just starting to understand what it is, and its relationship to stone,” she said in an interview Monday.

“We can remove a good amount of it,” she said. “But it doesn’t mean we kill it. We can’t do that. . . . And it doesn’t mean that it won’t come back.”

The film is actually a “multicultural” community of organisms living in the relatively harsh environment of the sun-blasted stone, said Federica Villa, a Milan-based microbiologist who has been studying the memorial’s surface.

The black pigment is produced by the organisms to protect themselves from solar radiation, she said in an interview Tuesday.

But does the biofilm damage stone?

“We don’t know,” Villa said from Montana State University’s Center for Biofilm Engineering. “If you read the scientific literature, most of the scientists correlate the presence of a biofilm with deterioration.”

But she said her experiments have shown that biofilm may have a protective impact on stone.

“To be honest, we have a lot of work to do,” she said.

The memorial was dedicated in 1943. The biofilm became noticeable less than a decade ago, the Park Service said.

The reason may be that air pollution, which can inhibit the growth of such organisms, has decreased, the Park Service said.

Or it may have something to do with the microclimate near the memorial.

There’s a small weather station on top of the memorial, and data is collected monthly, said Catherine Dewey, chief of resource management for the National Mall and Memorial Parks.

“Because we have this huge body of water,” she said, referring to the Tidal Basin, “is that tiny little bit of extra humidity . . . a factor?”

“We have some photographs from . . . 2009 where there’s very, very little growth,” she said. “In 2012, you see a little bit more. In 2014, it’s even worse. It’s grown immensely over the last few years.”

One day last week, Dewey tested cleaning fluids on small strips at the base of the memorial’s east side.

“It’s particularly bad on this memorial, for whatever reason,” she said as she sponged on the cleaning fluids.

“It’s really deep into the stone, so it will take some time,” she said. “What we’re finding is it may not be clean immediately. But give it six months, and it will be much lighter.”

Some repair work is planned for the memorial in 2018, and the grit on the dome could be addressed then, she said.

The 32,000-ton memorial, which honors the nation’s third president and main author of the Declaration of Independence, rests on 634 pilings and caissons sunk down to bedrock on the south side of the basin.

During the annual Cherry Blossom Festival, it is a prime backdrop for photographers.

Six years ago, a $12.4 million repair project shored up its poorly supported seawall, which had been sinking into the basin.

And in 2014, a large chunk of limestone fell from the ceiling. The ceiling was stabilized, and netting was installed in case anything else came loose, Park Service spokesman Mike Litterst said. A full repair has not yet been done.

The memorial turns 75 in two years.