I have a little bit of the same thing in my tank....took a piece into my fish guy to find out if it was good or bad....he said it's a type of sponge and doesn't hurt anything. Plus, it serves as extra chow for my sponge partial munching critters!
If you can't positively identify it and want to ensure that it doesn't spread I'd simply remove that rock from the tank. I never trust any uninvited guests that I can't positively identify.
Almost ALL sponges are beneficial and will help with water quality, by filtering your water. Your picture does show sponges, but I don't know the species. I have the same, or similar, sponges in all of my tanks.