Welcome to the forum, Travel Bum...
If you juuuuuuuuuuust did it, grab a sponge and clean it off.
If the thinset is dry, trying to sand it or scrape it will result in damaging the surface of the drywall. I'd patch over the non-tiled areas with drywall compound.
Tonto Goldstein….. but my friends call me Bubba
“You cannot build character and courage by taking away a man's initiative and independence.”