OK now I have more concerns. I saw the snake doing it last night and today again, and it seemed clear it's undergoing a shed. It has splotches of skin beginning to come off underwater, albeit fairly unevenly.
However my concern is that the snake looks really skinny. Like wrinkled, almost. Most of the other tentacled snakes look like this to some extent, they have weirdly loose skin compared to terrestrial snakes.
But a bit above his vent this guy's stomach is concave, and almost looks like it folds down into the skin to make a sort of > above his vent. I can't get clear pictures of this due to glare on the glass and his natural camo colors. I might be overestimating how severe the problem is, but it's hard to tell.
I'm planning on adding some larger fish (female bettas) to the enclosure as I've seen videos of tentacled snakes eating larger prey items than the guppies I feed mine, in the hopes that he'll bulk up. The reptile store I got him from said they fed them mississippi minnows before, which again would be larger than the feeder guppies I get. But there are a LOT of guppies in there at any given time.
Should I attempt to pull him from the tank and put him in a 20g for quarantine/feeding observation? I'm concerned that that would only stress him out even more.
I should note that these guys are rear-fanged and I've seen him strike at a pair of tongs I used to try to move him once. He's almost definitely stressed right now (sheds are supposed to be especially stressful for these snakes) and I really do not want to be tagged by something venomous. Not to mention that unless he's in the branches again, it'll be near impossible to get him out of the tank.