Sure sounds like bubble algae to me. Look like this?
If so, it's pretty common stuff. Really surprised your LFS
didn't have a clue. Don't pop them... the spores inside will spread like wildfire. You can carefully try to "roll" them off your rock with your fingers, but be gentle. Or you can take the rock out of the tank and scrub them off. You can also try the "scrape and suck with your siphon tube during water change" method to get them off. That works fairly well and if you happen to bust the bubble all the spores get sucked into the siphon tube and into your old water.