The aluminium of the calliper can react and bloom/corrode when in contact with the steel of the bleed nipple. Hopefully, being in contact with the brake fluid will stop this but you need to remove the nipple, and check the seat.
Change the nipple as others have suggested and you should be fine.
Could be.
Not strictly the thing to do but how about wrapping the threads with PTFTE tape?
Could be.
Not strictly the thing to do but how about wrapping the threads with PTFTE tape?