Ok, to revise my original question - I don't care WHAT it is... how do I get rid of it?! there's another small patch of it on the other side of the tank that seems (?) to be duking it out with my zoas. I much prefer my zoas to TP sponge, so I pulled out a small patch and did the 10% hydrogen peroxide dip for a few minutes and the bit of sponge that got dipped is not-obviously-dead. :-/