You may want to let them keep it in their bin for a little bit, they might let you claim a piece and leave it in there to cure some. If you do add it be ready with a water change because there will probably be a small ammonia spike from die off from the travel time from wherever they got it to their store.
CUC stands for clean up crew and it is made up of various snails, hermit crabs, shrimp... they will eat algae of the glass, rocks, sand and left over food and waste or decaying matter. You have diatoms now so you can start adding them. Check out the site that I pasted a few posts back it has a wealth of information on clean up crews and you can order from them if you want but you could just get them from your LFS.
What are your latest water test readings btw?