That looks like cyano not diatoms.
If you have cyano then you have excess nutrients .. primarily phosphates. While your phosphate test kit may say you don't have phosphates .. remember that its only measuring whats in the water column at that moment .. your cyano is sucking the phosphates out of the water column just like a phosphate sponge .
In general you want to maximize the exports of phosphates while limiting the import of phosphates.
Std measure to control phosphate include water changes using RO water, use a phosphate binder (phosban etc), wet skimming, reduce feeding and make sure the food your using doesn't have alot of phosphates .. flake food is loaded with phosphates - frozen food should be thawed and rinsed before using, clean filter media daily, kalkwasser to precipitate phosphates, and use of macro alga's to compete for the phosphates. Manual removal of cyano will remove the phosphates within the cyano.
Unless cyano is smothering corals its not going to harm anything .. if you want you can view it as an inexpensive phosphate sponge .. let it grow and just siphon it out removing the cyano and the phosphates with it.
Edit
Most cyano threads bring up water flow as a cure .. thats only important in the context that high flow helps deny nutrients and allows the skimmers/filters to remove crud that would otherwise release phosphates. Cyano itself doesn't care about water flow .. blowing around the cyano bacteria isn't going to resolve the problem.
