After you clean it, the grading service will probably not grade and slab it. I had that happen several times on other US silver when I was collecting in the 70's. Some of them were cleaned when I got them from a coin dealer, but I was too new to know the difference.

Your Columbian half will probably bring less than $20 anyway, so your may want to go ahead and clean it if it's just going to be a souvenir. There were a lot of them made, and most of them circulated.