It could be either and the only way to know for sure is to take a glass of your tap water and drop an air stone in it and watch what happens.
It could actually be residue from a cleaning, could be trace oils from the rockwool or sponges you are using for grow medium, could be adhesive residue from those lables...etc.
Is it a problem?
Btw, what nutes are you gonna use?
i am growing 3 different basil varieties ( no shit ) in my aerogarden 7 right now using a 1/4 strength dynagro nute and these babies are loving it. I pump in external air...experimenting.