Cyano :(

[Myka]

Just follow the instructions on the box. You may need to re-dose again next week or so.

Fwiw, your tank looks quite mature, how old is it? Your rocks also don't allow real good flow around them. I think it's time for a revamp. Cyanobacteria is VERY good at utilizing nutrients and can even bind nitrogen, so even a very small amount of nutrients in the system is enough for cyano to flourish. I have a feeling there is a fair bit of nutrients in your tank, but the cyano is utilizing it so it's not in the water for you to test.