I am quite sure your goldfish are safe. We use sodium percarbonate (think dry H2O2) to clean our ponds and I would say they almost enjoy the bubbles.
As far as the source of micro-bubbles, this can drive you to madness. The most likely source is a faulty gasket/seal, and then come pinholes, pockets of air trapped from the last cleaning to the most serious, some decomposing crap (critter ) stuck in a filter somewhere.
Anyway, you need to find it, though probably innocuous, it can be the warning of a serious failure. :gw
Well have fun, remember this is a hobby, not, well… Golf.
No sweat then, the air stone is doing what air stones do.
If the bubbles are noticeably finer then the holes are plugging up and replacement time is near at hand.