I know this sounds dumb, but maybe he is protective/obsessive.. over you or your daughter? I had a gelding that did not like to share me. If he was in turn out and I was with him, if another horse came over he would chase them away or would put himself between me and the other horse. If he was in his stall and I was feeding him treats, if the horse next to him tried to nose for treats, he would charge them through the stall.
So if you were giving him attention in his stall and another horse tried to get attention, that could explain why he does this. Do any boarders/workers notice this behavior when you are not there? Is the mare new to the stall? Has there been a stall change and he just doesnt like her. Perhaps speaking with whoever works with the horses could shed some light on it. We have a standardbred mare who just plain hates having neighbors. She is in a secluded stall, (she can see and be around horses, just none next to her or behind) but during feeding time she will still kick or bare her teeth. We also have a gelding who is not happy with neighbors, but is ok until feeding time, then he tends to kick the stall wall. Some of our other guys get unhappy when a horse next to them gets treats, but they don't. So it all depends... but I would ask the workers if they notice anything.