I suppose it's a matter of balance. I feed heavily once or twice a day with both live and frozen foods. I have not had any issue with diatom, cyano or hair algae. I run GFO and activated carbon in my tank which keep the nutrient level low in the water column. I suppose the algae in my refugium is helping as well.
Diatom growth can be limited by the shortage of soluble silica (silicic acid or silicate.) DI resin removes soluble silica in your source water. Is your DI cartledge exhausted? If not, it could be your salt mix or food.
A Florida fighting conch, Strombus alatus, will keep your sand bed free of diatom (they also keep your sand bed stirred and free of algae in the shallow part of your sand bed.) Sponges also consume diatom as well. So do some planktonic algae and protozoa. If diatom is a constant issue in your tank, I highly recommend adding a Strombus alatus. Queen conch (Strombus gigas) grows too large and it quickly starves to death from the lack of food in our aquariums. Strombus alatus will also starve to death if your tank is very clean or if you add more than one of them to your tank.
Cyano can be a problem even in nutrient limited SPS tanks. It can be from overfeeding or a water current issue. How is your phosphate level? Cyano can assimilate nitrogen from the air, but it needs to absorb phosphate from the water.
Carion feeders such as Nassarius snails quickly consume left over food on the sand bed and keep uneaten food from decaying and increasing the nutrient load in the water column. I see them coming out of sand as soon as I add food in my tank. They stir up the sand bed as well and keep both cyano and diatom from settling there.
Tomoko