coral has bubble on top:(

[jenkins2212]

I bought a green grass mushroom about two weeks ago and it seems to be doing as good as it can do under 36w pcs. But at the top of the coral there is a bubble growing from it. I dont know if this happens to these corals or not. Ive read that it can be stress, but all other corals in the tank are fine. Any help is appreciated.

Thanks
Travis

 

[ralph atl]

pics & all parems please........