Yeah, it's pretty bad. Your batteries might have exploded, or may be leaking. Depending on how bad it is, you may have to see if your mouse still works. I don't think it's the brand (unless it's happened madami than once), they may have just been bad batteries or 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, or they could be leaking acid, either way, rinse your hands off, just to be safe, and dont use those batteries again! are they burning or discoloring your skin?