It should be alright, as long as it isn't in the way of your bp if it's headed for the other side of the enclosure or the water, because some bp's (like mine) hate getting wet, even if it's only a little bit damp, so I wouldn't want to make the bp have to choose between staying dry and warming up, but that's a very minor concern, so I think moss in the enclosure is fine as long as you change it every now and then to avoid bacteria formation.