Is it more of a slime or is it more spongelike couldn't really see pic on phone but I'll check out on computer when I get home if It is spongelike it could most certainly be a black sponge. They and other colors can pop up In tanks naturally.
I've never heard sponges being bad but don't quote me... I have some pink and yellow sponges that popped up from nowhere and they don't bother anything if any thing since its a filter feeder it will help on cleaning the tank
Good litmus test is to try to blow it off gently with a turkey baster. If it comes off, it's likely a black cyano. If it doesn't come apart, then sponge would be my guess too.
