Is this stuff that appeared on your rocks Hard, Soft, Spongy or rubbery feeling? Sometimes just looking isn't enough. Don't be afraird that you'll hurt it, or that it could hurt you. You're safe there! It may help in identifying it so that you know whether to scrub it off, or leave it on there. Sponges are good things. So give it a good lookover before you decide what to do with it.