If the clowns spawn, unless you take heroic measures to isolate and raise the fry (involves installing Frankenstein's lab in your house) the fry will just become part of the 'circle of life'... fish and coral food.
Sponge is fine - filter feeder. If it grows too crazy you may want to remove some of it but for now it's small - let it be. I've got one kind that pops up occasionally that's black and stringy and can get rather unsightly - but angels love to pick at it
I have those sponges in my tank too. Every now and then I have some pop up. Sounds like your other tank critters will be having some live food soon. I know that it is sad but, clowns reproduce quite a bit.