My reef is almost 20 years old and I still get occasional small patches of diatoms and red algae. So does the real reef. In my case it is because I choose to have tons of coral and very fat fish. When I get sick of it, I polish the water with something like Polysorb and make sure my RO
filter is working. City water has selenium In it and my understanding is that can promote diatom growth.
One note, I have noticed that coral that is healthy rarely has any algae of any kind on it. But if polyps get sick they get covered fast.