Yeah, it's pretty bad. Your batteries might have exploded, o may be leaking. Depending on how bad it is, tu may have to see if your ratón still works. I don't think it's the brand (unless it's happened más than once), they may have just been bad batteries o something, I'm not sure.
i dont have a definate answer for you, it could be something as simple as someone spilled some water and it got into the mouse, o they could be leaking acid, either way, rinse your hands off, just to be safe, and dont use those batteries again! are they burning o discoloring your skin?