I had a bubble coral that did this. It would do it every once in a while. Sometimes it would spew a brown snot like substance. I think it was just ejecting waste. However, mine would also do this if it was upset, if the salinity changed, if you had too high of flow where it was. I found it to be a little tempermental. If it comes back out and looks good again for a few days you are probably fine. If not, you probably have something going on in the tank that is aggravating it.