Uh-oh! Nikita, do a Google search..."Black beard algae".
Hey now... You may have just inadvertently diagnosed my tank with BBA which I've been (unknowingly) monitoring for some time now as it's grown on my sword leaves. More startling than that, however, is the surprised tone in your post, and the use of that
expression isn't assuaging any fears you've now instilled!
WHAT HAVE YOU DONE. WHAT DO I DO.