Whats Wrong With My Bubble Coral?

[theROYSKIE]

so i have a bubble coral that has been thriving in my 24 gal nano cube with the viper 150 watt mh 4 approx 4 months. there is also a huge branching hammer and a little frogspawn frag in the tank along w some shrooms and star polyps. everyone is looking healthy except the bubble which has been deflated and sucked in 4 approx a week. i target feed all the coral with DT'S, MARINE SNOW, PHYTO PLAN, AND THE KENT LINE OF CORAL FOOD(ZOOPLEX, PHYTOPLEX, CORALVITE, CORALACCEL, CHROMAPLEX) i do weekly 8% water changes, i mix my own water with seachem salt mix and my r.o. is putting out clean water. ph-8.3, nitrates-10-15 ppm, specific gravity-1.0255, salinity-34.5 ppt, temp-approx 76 degrees f.
i hAve no clue what is going on. last week the bubble caught and killed a lemon peel angel fish, it didnt eat it but it tried-POSSIBLE REASON?
the actual flesh of the bubble is starting to retract around the perimeter. does anyone have any insight to what is going on? and what i can do to try and save it?
this is the coral today...

the carnage last week, coral was still visually healthy

this was 2 weeks ago

 

[weaselslucks]

you sure the fish didn't die and he just happened to catch it?

 

[theROYSKIE]

i am not entirley sure but the angel was seemingly healthy it would readily eat frozen foods as well as pellets but there is the possibilty. but would that hurt the bubble coral? fish alive or not?