my encrusting sponge/coral? is changing colors

I got this rock with yellow zoas and an encrusting something or other on it, I was told at the LFS and on this forum that it was a sponge, but it feels pretty hard and has lots of little polyp like things that extend and contract when you touch them with anything...so it is coral or sponge and whys it changing color, its turning darker in color and had been growing fairly well, filled in a small hole that was in it and is spreading nicely, doesnt seem to bother the zoas most of the time though...so what the heck is this thing...

 

that looks more like a montipora or porites

 

yeah, I started thinking about it and was like "sponges dont have polyps" is there anything I should ad to the water for them to be healthy, and is getting darker a good thing, they where whitish yellow and are now getting darker. all polyps extended every day so far

 

That means it's getting healthier

 

looks like porites, the white color is from it being bleached in that area, the brown color coming back is the zooanthallae repopulating that area in the tissue

 

good deal, so will it grow just with the lights, or do I need to feed these...the whole rock was 45 bucks, so I guess I made out ok, two different colors of zoas, and now these...my first hard coral and i didnt even know what I had...

 

Im curious about the polyps on it.... Got a better pic of the rock? Move the rock close to the glass and take a pic of it?

 

+1 on porites, good score! and you shouldnt have to feed.

 

geekdafied, what I am calling polyps are little clear tubes with a little crown of tentacle things at the top...they are pretty small and wave in the current, they extend maybe an 1/8 inch at the most. sometimes in the morning one will be touching a zoa and it wont open, so i just blow water on it and it retracts then the zoa opens and then it comes out and all is well...so they dont seem to be agressively hurting the zoas...

 

zoa's are commonly referred to as polyps. Im in the habit of saying that cause at work thats what everybody calls them. Its a reference to zoa's and palys. Im interested in the zoa's you have, but I wanna see a pic of them. We might do some trading if you're up for it....