Man, I lost the chance to post my important question to that question!?!?!
Mucous membranes are the moist linings of most body openings (mouth, nose, genitals, rectum, eyes). The sort of exposure you describe has zero chance of HIV transmission; you have to be exposed to much more blood or other infected secretions than is remotely possible by scratching dried blood on a coin.
If this situation is "really worrying" you, I assume you have stopped crossing busy streets, will no longer be riding in automobiles, and plan on never swimming again. All those activities carry far higher risks of death than handling coins or other things that might have blood or body secretions on them.
HHH, MD