Brown stuff is normal but it does not look normal. That gaping mouth and flaccid tentackles do not look good. Perhaps it's just eliminating but that should only take a few hours. Get another picture in the morning...
here is a pics of mine about 2 week ago, not sure what was happening but everything look fine so far. I got answer like dying, releasing eggs (spawning), releasing waste, expelling its zoos. I would just keep watching its health, ck water parameter's and do a water change.
That looks like sexual spawning to me...
However, the green 'nem looks to be on its last "tentacle" so to speak. However, since it's really new, it might simply be shocked. Has it ever opened fully?
Mine seems to shrink a little everyday also but not as bad as yours. Have to tried to give it a little piece for food like shrimp (small piece) yet? Mine seem to perk up alittle after I do so.