Yellow polyps are disappearing

[Dean29G]

I was wondering if anyone had an idea why my yellow polyps are slowly disappearing over the last 8 months? My water tests are all pretty good and the lighting is only about 3 months old.
Some info on the tank:
29 gallon, 110 watt PC, moon lights, 15 gallon sump and refuge with 18 watts pc lights, ETSS 500 evolution DIY skimmer, 1 yellow tang in tank about 1 month and 1 yellowtail Damsel about 2 years old, blue leg crabs and scarlet leg crabs, msc snails.
Calcium - 420
DKH - 9
Ammonia - 0
Nitrates - 0
PH - 8 - 8.1
Salinity - 1.024 - floating type tester- not sure if it accurate as refractometer

I am wondering if I don't do a lot of water changes, if my trace elements are really low. Would that affect it greatly?
Any info would be appreciated.

Dean