I noticed similar fluid filled sacs in a mystery orange acro when I first started keeping SPS about one year ago. At the time, I wasn't dosing anything except dkh when I performed water changes. My other sps seemed to be doing fine - maybe a bit starved for the correct water chemistry - but still growing normally. This acro did poorly in both my and my roommates tank, but once it was moved to his much smaller tank, it no longer got the fluid filled sacs on it. It was only recently, when I started tested for and dosing everything, that I realized the coral was starved for magnesium. Magnesium dosing caused the polyps/skin to regrow correctly. Magnesium levels weren't incredibly low, probably 1150-1200, but I hypothesize that this coral requires more magnesium to build it's skeletal structure. Without it, pieces are missing, the coral's skin doesn't correctly attach to the skeleton, and as a result, bubbles form.
The bubbles were about 1/2-3/4cm in diameter. In 12 months, they never spread to any other corals. I have pictures floating around somewhere. If you are interested i could try to dig them out.
Might be relevant: I probably broke the tips off of this coral more than a handful of times before the bubbles, but I have broken it many times since and the bubbles have not returned.