I noticed the same thing happening to mine when I was watering this evening - can't say it has ever happened before - no idea what is causing it.
Looks like the garlic equivalent of "leek rust". Get this myself most years and does'nt harm the bulbs. Also like leek rust can be washed off or outer leaves discarded before eating. Not poisonous so dont fret. All comes down to funny weather conditions i guess
Thanks for the replies, I can sleep a bit easier now :-) Looking forward to the first garlic harvest!