Help with ID. What is this?

[kattycake]

I can’t figure out what this is. Is it brown slime algae. It’s just this one spot. It looks almost like mold. What is it, how do I make it go away? And what kind of problems will it cause?

 

[SAPPER]

Its sponge and its fine

 

[kattycake]

Its sponge and its fine

Wow. Thank you. I have been googling for days... never thought sponge at all! I have a bunch on neon green sponge, but somehow it never clicked this could be too! Greatly appreciated!