Looks like some kind of a sponge type growth. Is it growing. You get lots of little spongy type things growing on live rock, specially in darker areas. I have this greenish-grey-yellowy stuff growing on one rock where its darker. Looks like a big scab. I dug the rock out to make sure it wasn't mold, cause it sure looked like it. Nope, its sponge. I have a few that look a lot like yours. After a while they turn a light tea colour. Mine don't get big and they don't bother anything