How long have you had the skimmer running? Did you buy it new? I'm currently running a CSS 65 and it produced microbubbles like that for the first week or two even though I bought it used.

Have you added anything to the tank recently (large amounts of food or additives)?

One thing you'll probably want to do to the skimmer is make the input tube shorter. The closer you get the pump to the surface of the water, the better it will skim.