coral dying?

[Flash]

Forgive me in advance for my lack of terminology!

I got home last night and a coral in my tank looks horrible. All other corals are doing great all open extended and such. But this coral was shrunken and turned from yellow to a greenish color. It's usually fluffy and has a bunch of polyp (I don't even know if that's the word I am looking for) like things comin out of it.

Nothing in the tank has changed, nothing new has been added. It is usually the size of a toonie and now it's between a nickle and a quater.