Algae outbreak

For balance I am going to show you the dark side of this hobby. I have had an unexplained outbreak of algae. Nutrient levels have never been lower, so I just don't know what the issue is. It could be the salt and I wonder if the fact that all my snails have been eaten by the jade wrasse as exacerbated the problem?

Anyway, I am resolved to rectify this problem & will eliminate potential causes one by one.

One of nevs Frags coming on but look lower....