I am no expert. Stoney Bud would probably know best.
My opinion is that if it is not grey mold on the stems or leaves (caused by Botrytis cinerea); it may be caused by the high humidity level in the reservoir. I would clean it off and make sure it is not on your stems or leaves.
Add 5ml hydrogen peroxide for each gallon of water. Clean the mold or algee off and keep temp of the water around 68'F. Humidity will be high in your reservoir, of course, because it has water in it... What is the color of your lids? The darker the better. I prefer black. Light entering your res will encourage algee growth.
Hey Ad1, good to meetcha. Mold grows from spores. When you're going to work on your reservoir, make sure you wash your hands real good first. Try not to have on clothes that you've been out in the woods wearing. Bringing spores into your grow area can decimate a crop.
I would wash the lid with a mixture of one teaspoon of clorox to a gallon of water to remove and kill the existing mold. It'll make it harder for mold to grow on it as well. After washing it, just drain off the water and put it back on. No rinsing should be done.