Algae turning yellow under lighting?

I can't post a pic yet but I had 3 bottles of nannochloropsis growing well but after 5 days 2 have started to turn yellow. Is it dead or what happened ? I have 1 500ml bottle that looks great as well as a 2 liter that looks great. Should I start new cultures from the 2 liter ? Refertilize ect? Can it be refrigerated after its grown out and if so how long can it stay good.