from what I understand they will do this and there are many factors that could be involved: hunger, age, bad water chemistry....
The most common factor is the food supply and with two in there you might be cutting close....just cause the sand is dirty doesn't mean there is food for them. Some of that may be algae and they are not really algae eaters. In fact, I don't think anyone is really positive about what these guys eat in an aquarium....but we do know some of them will take to meaty foods, snails, worms, etc. They thrive on anything living in the sand but in a small tank they can devour that supply pretty quick. They can really deplete your sand bed of life but man they keep it clean!
I haven't seen a worm in my tank since I put him in there. Coincidentally he is huge! Check your PH first and the the rest of water chemistry, if that is in check try bribing him with some mysis.
Also, notice teh second star you have in there. is he getting bigger while the other one is losing an arm? They could be competing for food and one of them is obviously losing.
Good luck.
