A loo mask is essentially the same as a domino mask, but it's the lower-class version. It is the mask that Zorro and The Hamburglar wear. This sort of mask might also be called a half-mask or an eyemask, but these terms are fairly obsolete.
The name comes from a corruption of the French loup, meaning wolf, and refers to the wolfish nature of those that wear it, rather than any aesthetic pretense. It may be shaped in the same way as a domino (à la the Hamburglar) or simply be a strip of black cloth with a couple of eye holes cut in it (à la Zorro). Regardless, it covers only the top part of the face, specifically the eyes and nose, and is not adorned in the way one might a masquerade mask.
A loo mask does not need to be used in the pursuit of nefarious lurkings and pouncings; they might also be used to protect a lady's complexion, as was popular in the 1700s, or be an fairly non-suspicious means of providing anonymity and/or costuming. These days they are most commonly associated with outlaws, so you should not wear one to your local bank.