Translation:Dych chi wedi ymolchi?
Wouldn't this be "You are washed?"
Yes, it could be that or 'are you washed?
It would depend where the emphasis was in the sentence.
Dych chi wedi.......ymolchi? = 'Have you washed?'
Dych chi.....wedi ymolchi? = 'Are you washed/You are washed'?