If your clown is "loving" your coral this probally be the problem
I've had to remove an alveopora from a tank because a clown decided to host in it so eventually it would never open.

does your rock have a lot of flow around and under it? detritus build up can be a big issue
Premium fiji tends to crank out alot of detritus for 2-3 months so that may well be the problem. I would use the filter sock occasionally and then blow off the rock with a powerhead. I do this every 1 to 2 months on my 215 to keep detritus from building up too much in the pores of my liverock. In between I don't use any type of mechanical filtration.