It started out as a small green spot, maybe algae or sponge. At first I liked it but now it is taking over my tank, please help.

I have a 180 gallon tank, I just added 180 lbs of live rock. I have 20 snails, 15 crabs, 3 shirmp, 2 damsle and a clown fish. I havent added corals yet I am waiting for it to be stable for a while. I added my rock about two weeks ago form an established tank off of craigslist. I notices some of...