I would call that bearding. I would do an inspection and check to see if they are crowded and ready to swarm. A super won't hurt if you aren't sure. You can also put a small stick under the corner of the lid to prop it up a little and increase the air flow.

See if there's a little hole there. Bees can become very concerned with either closing up a little hole or opening it. getting in or getting out. Or at one time there may have been a way in there, and there trying to figure out why it isn't there anymore (if you had another super there at one time).

If it is hot out and there are a lot of bees in the colony,they will beard outside so the inside temp does not get too hot. Often propping the top up a little helps releive the heat and lessens the bearding.