It's very difficult to see, but I guess we're looking at a horizontal crack just beneath where we would expect the "built in T joint" would end. It's very common amongst Yonex racquets, particularly for the older type of metal T-joints.
I'd love to say to you that this is nothing to worry about, and most of the time it isn't but I have had a Carbonex 21 which started off like this, then one day the whole head came off during play.
I don't think it is serious yet. Usually the cracks would tend to get larger, then flakes of paint will start peeling off before you see any major problems affecting play. Just carry on as normal and monitor it. There's nothing you can do about it really.