Does it have an almost greasy film on the top?
You can catch it now if your quick, before root rot really sets in....
Dump your res, clean out the whole system, fresh water, fresh nutes.
You don’t need to add pool shock or h202 etc. once you have things under control again,
You’ll just have to figure out what’s caused it in the first place. Eg. light leak in res, not enough aeration in water, water temp too high.
It’s funny how people use pool shock etc as a preventative, you don’t need a preventative if everything is running right.