It may be reacting to something it doesn't really like. Or did those heads just catcgh something and are eating? Mine close up tight when i feed 'em. Could be many things. First knee jerk reaction is , test parameters, do a water change and then run some fresh carbon. Might not be much of anything to worry about. It will close up from time to time. As long as it doesn't do it for more than a few days at a time. There are a couple heads that are really closed tight. So keep a close eye on em. How long have they been this way? Are the tight heads completely white and skeletal looking? Or do they still have all their teal color?