purple sponge

I dont know if this is the correct place to put this topic but I didnt see a sponge headline. lol  I was messing with my tank today and I have purple sponge growing in like 6 different areas....   I thought I just had it on a tonga rock that I had got from a guy that sold me his tank but it must have gone crazy when I wasnt looking.
I was at a fish store in orlando recently 2 wks ago and they were selling it $40 for a small square.  I must be doing something right for it to take off growing..  If I am wrong in assuming this let me know. lol