I looked again, and I think he is just growing an arm! The other one I have did this while hiding in one of the aquatic plants, so I didn't witness the process. Luckily, it's taken me about 3 hours of research to find this article:

http://www.suttonmass.org/animals/fr...dpolearms.html

...in which time I can now see his elbow peeking out. So, I think all is going according to plan. Why did it take me 3 hours of stressing to find that article?!?! Jeez. I had no idea they grow their arms out of a little hole in their sides! Craaaaazy!