The very first WTF I had would sleep with her nose pressed up against the lid of the tank and it caused similar color marks on her nose (though they where bigger in size) If nothing is wrong with the wire-mesh lid then it could simply be him rubbing his nose against it (I'm not sure how you would keep him from doing this) ...though to help prevent the possibility of accidentally hitting him with the lid I would lower the stick he is sleeping on down a little bit.