OH Snap!! We bout to get the all new Ultra rare BOUNCE Forest Fire Digi. Only $2k per inch!!!
Just kidding of course...
In all seriousness, I've seen blisters under SPS coral tissue as a result of heavy ozone use or tin/heavy metal toxin exposure. Those tend to look more like textured blisters than smooth bubbles though. Hard to say from the pic, it almost looks like growth nubs, but I'm sure if you're looking at it in person you can tell if there is just fluid under the bumps or hard skeleton. Is your pH running super high? Sometimes that can cause tissue inflation. If the digi remains growing and otherwise healthy, I wouldn't stress too much. Best of luck!