Well... it's something most folks would rather not have, but it's not *really* bad. I mean... there are far worse things to have. But the problem with bubble algae is that once you have it, as far as I can tell, you'll always have it.
First thing - don't pop the bubble. The spores inside will spread the stuff all over the tank. If you can remove the rock from the tank and scrub the stuff off - that's your best bet. But most likely, you won't get it all and in a couple months it'll be back. Another thing you can do is to gently "roll" the ball off the rock with your finger. Sometimes this works, sometimes not and you end up popping the bubble. Oops. The other thing you can do - which I've ended up doing every other week or so - is when you're doing a water change just go at the bubbles with a siphon tube and scrape the bubbles off the rock with your siphon tube. If they pop, all the spores just go in the siphon tube and out of the tank. I don't really have a spot in my tank without bubble algae nowadays, but I just do a little "weeding" every other week and it keeps it at bay.
I've battled the stuff for a couple years now, but it's not a big deal in my book. If you let it get out of control, it can crowd out some corals, but it is kinda pretty. Some folks have had some luck with using Emerald Crabs to graze on the baby bubbles and keep it under control.
Here's a good article on the stuff:
'Bubble' Alage: Selected Descriptions, Controls and Comments by Horge Cortes-Jorge, Jr. - Reefkeeping.com