Are they getting covered with mold and dying or dying then getting covered with mold?
That kind of looks like the mold you get when uneaten food is left in the tank. How much are you feeding?
Could you give some more details on the tank and its inhabitants?
Tank size, how long its been set up, water parameters?
How long have you had the fish?
Anything new introduced or are you doing something you havent done in the past?
Water change schedule.