Hello Teresa and Welcome to the forum. If you do not want to open it, what I have done is leave it outside for a couple of days (out of the sun) then bring it inside and put it in a paper bag with a box of baking soda (seal bag) for a week or so. This has worked for me in the past. I have other melodicas to play so if one is out of commission, the wait does not bother me. If you decide you want to open it, just be careful with seals and reeds. Use some mild soap and water and dry it good. I think someone on here actually opened it up and put it in the dish washer.
Monsters of Melodica