Hammer coral question?

I have a nice green hammer coral. I have had it for about 4-5 months now. last week I noticed he seemed to be smaller aka not extending his hammers out. One other observation I noticed is that some of his hammere are extended longer like 2 inches further than "normal". I have made one change to my tank. I added active carbon in a Media reactor to help cLear the water and help water chemistry also. I have noticed a raise in the po4. Anything else needed let me know.