Coral losing color at base

[AV30]

Hey everyone,
I am new to reefing and my tank is about 6 months old. I have some some corals in my tanks that are doing well ( star polyps, toad stool, rock anemones) my water tests out good and has been stable (temp:78, pH:8.2, ammonia:0 nitrates & nitrites:0, Alk:10, calcium:420). I did add some emerald crabs recently. Anyone have any ideas what is happening to my coral? I am running a 100 gal IM ext tank with 2 Kessler 360X lights and the coral is placed in the middle of tank with medium-high flow.

 

[CazaCorals]

It could be off parameters, the crabs, a fish, or lack of light in that area if the heads of the coral are too big,