Live sponges? What kind is it? There are some that are known to be covered w/ algae and such even in the wild, and don't seem to be harmed by it (the one I am thinking of is an orange or yellow ball shaped sponge.) I would think that you would run the risk of damaging the sponge if you attempted to removing any growth, but leaving the sponge covered could be just as bad... Perhaps a very soft and thin artist paint brush? I assume you already tried waving the sponge in the water and the output of a gentle power head?
