I see three sets of leaves, try a 1/4 strength feeding regimen (earlier I said feed by the first set of leaves with ridges, which is possible to do, I wanted to play it safe.)
I've seen a lot of things growing on grow plugs, green stuff, white stuff, never really bothered anything, as far as I could tell. It can be a little expensive, but something I liked to use in a hydro mix is
hygrozyme, I don't remember exactly how, but i believe it takes care of stuff like bacteria.
Here's some stuff about
hygrozyme, "Premiere horticultural enzymatic formula. Enzymes are natural and environmentally friendly. They are catalysts and speed up chemical reactions. Enzymes in a growing medium eat dead protein removing areas where illness develops. Through this breakdown process plants are better able to obtain nutrients. More nutrients, less attacks, allowing plants to grow bigger, stronger, and faster."
How far is your light from your plants and what's your air flow like?