This is a pretty severe shrinkage, I'll also point out that the pic shows a significant loss of zooxanthellae, least that's what it looks like to me.
I would check your water source, run some fresh carbon (fill er right up) for 2 days and a polyfilter if possible.
There are several things that can cause shrinkage like this. Chemistry off is one, copper, temp spikes, pH swings, heater problems, getting hit directly with buffer, possibly even another coral or anemone. I've seen similar events when the ammonia spikes.
Check your filters for buildup, skimmer for output, make sure the heater isn't going wacky, if glass heater check for condensation inside the glass, and check the wires and seals going into the heater.
Keep me posted
Thomas