Mine finally dug its hole under the rock. He doesn't fully come out of the whole at night though. He sticks his head out and proceeds to eating. Will he ever come out fully and "mop my sand bed" or just stay near his hole?
From what I've read this is a common behavior, as long as enough food exists near where it has decided to call home there is no need to leave the safety of the spot it has chosen.
they are also known to go into hiding when they are splitting, reproduction through fission is also fairly common in healthy cukes.
graveyard worm how am I supposed to make sure it's eating before I purchase it? Doesn't it eat at night?
Mine come out when they are hungry, they are typically nocturnal but not always. Its hard to describe, but a healthy one looks that way. If there's white tissue exposed at either end then there's a chance its not heathy ( like the previous one you had ). They should also be hanging onto the substrate, LR, glass etc., with their suction feet, if its floating around the tank at all then stay away from it. If you have any doubts dont buy it.
I am really worried that it's not going to have enough to eat in my tank.
This is a possibility a 40 breeder is probably marginal in size to have enough food to sustain a cuke.
Do they only eat Algae? Or do they eat left over food as well?
I belive the article stated that there was atleast one which was specifically an algae eater, chances are pretty good that there are more. In a smaller tank these may require supplemental feeding, and should probably be avoided unless you are ready to commit to culturing algae on tiles or some other substrate to feed it. I doubt it will eat any prepared food, I really dont know, and even if you were to culture food for one I cant say for sure whether you would be able to feed it enough. There are ways to get desireable algae to grow on your sand bed.
However it appears that many of them will eat a variety of stuff from ( detritus, fish poo, wasted food ), and with a little overfeeding you should be able to keep one.
Its kindof a crap shoot, chances are that not enough work has been on the various species to know exactly what they are eating, and even if you knew what a particular species ate, finding and identifying one at an LFS would be a challenge in itself. If you are uncomfortable keeping one then you may want to change your mind and not get one, for sand bed cleaning choose a fighting conch or 2, some cerith snails, and some nassarius. These are intrigueing in their own right. Good luck in whatever you decide. Keep us updated, and dont be afraid to ask any questions you might have.