dieing duncan

[Aquaria]

Is this mold? Just started receding about a 2 month ago and 2 days after my last w/c this happened temp is 78 salinity is 1.025 other per should be stable I don't have a lot of algea growth or cyano anymore since I caught up on my w/c I change 5g every week to week n a half. Could old lights cause this? They are due for a change out. Ducan has been in my tank for a year and was doing good when I moved back in Oct my nitrates were up to 25 and it got a head crushed and it didn't even faze it. Seems weird for it to just up n die now that things are good