rock is another thought ,
I just googled what would turn my new saltwater tank green overnight ,
the three results I found are as follows :
some salt mixes contain phosphate (instant ocean is phosphate free) so this aint it .
is there a outside light source that hits the tank (uv
rays can cause water to turn green instantly) cure lower blinds to reduce light .
some limes they use to make dry rock can react to salt water causing a bloom , (this was your thought)
ok. I'll check the heater. I never would have thought of that. And I am going to smack you. 100% water change?
lol I would have done this already but after reading what I just did it would have proved it useless