Shouldn't it be some short form of Michahel? Maybe Michus Mus?
In Spanish Micho/Miz seem to be colloquial for cat (Gato)? wth. I think German Miezekatze (Kitty-Cat) must be related to that somehow.
Latin mico, glimmer. Maybe the eyes in the dark??? Etymology of Spain words is lacking in wiktionary. At least on English wiktionary, so I am guessing here. And this has anyway nothing to do with a mouse or Michael.^^