Hmm, not cool that misting. Claim the whole tach is faulty and demand a new one, as it's not properly sealed under warranty? You could check the cables as these are where the moisture might be entering.

If no dice wait for it to dry, open the back of it up and put in some of those silica gel packages you get in nut packs then seal it up with high quality glue/waterproof gaffa etc, and hope it doesn't happen again.