That looks like lice to me, but that's strange that your others aren't showing symptoms. Whenever I have a goat with bald patches, I lather them in Nu-Stock ointment. It is a sulfur cream, which tends to help with lice too.
Billy block's don't do enough for them, they are basically a treat.I leave a billy block in there room they seem to lick it I just put a new one in there the other day.
Where can I find copper bolus I called my local tractor supply they said they just have a copper supplement that's in tablets