Water probably pretty good if thats Xenia thriving in the background.
Elegance corals can be really finicky. Make sure flow and light isnt too much.
You could do an ICP test from ATI and see if anything really wonky in the water. If nothing, look into medicated dips maybe anti biotic, ciprofloxacin is a good one.
Google elegance disease too to eliminate that. Fresh water dips there are a last resort.
All and all, these guys take a while to actually die. So check one thing off at a time that it could be and treat, start with less drastic measures, and dont do anything too fast or drastic unless you must.
It may just going through a fit.
You could manually remove the nusiance algae and run carbon to start, and the ICP.
