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Looks like it could be black beard algae , if so scrape as much as you can off glass and use Excel (glut) and spot treat it . Google BBA and there is a ton of info on it .
If you don't have one, you could invest in a scraper (I have no clue what it's actually called). I got one to take care of about a billion different types of algae in one tank, and it just took it right off. If you have a glass tank, it's really nice because it has a razor-like blade on it, and takes the algae right off.
Yeah, I've used that one, and it doesn't do me any good on most stuff, just kind of helps scrub small stuff off the wall. The razor one is the way to go, IMO. Just be sure to keep the entire blade on the wall, otherwise it won't work.