Cyano with no nutrients?

[Proteus]

In the last week I have had a slight cyano issue.

It started on the bottom which is not sand but crush. And only enough to cover glass. I vacuum it regularly. I don't have any noticeable phôsphates and nitrates were 0-5 ppm.

I blow it off the rocks daily and it's not even really there just slight amount of red tissue and the air pockets. I use rowaphos and prodibio. I feed my tank very little for the most part spot feeding my two fish and 2 shrimp. I run a mce 600 skimmer which does its job fantastically. Also run filter pad @100 micron changed daily

Can anyone explain why this would be without any noticeable nutrients.