Mine does this every so often. When he does he lets out this string of gooey stuff. Pretty nasty looking. Then a couple of hrs. later he pops back out. The 1st time I saw this I though he was dying and did a search on this board. From what I've read this seems to be pretty normal. I would keep an eye on him to make sure that he opens back up. You may want to check your water to make sure that everything is in check. Also good lighting, and good oxygen levels help.