Montipora fungus or sponge?

[matt r.]

Wondering if anyone might know the answer? I'm hoping it's just a form of sponge? Doesn't seem to be having any negative impact on this piece of monti, but I just don't know where it came from or what it is? It's yellow, soft to the touch, very thin and spread out.

 

[matt r.]

By the way, it's only on the underside of the montipora