why is it golchi'r and not ymolchi'r in this sentence
Golchi is to wash something, ymolchi is to wash yourself. eg dw i'n golchi'r tatws = I am washing the potatoes, Dw i'n ymolchi = I am washing (myself).
The ym- prefix usually means that the action is being done to yourself:
I don't think you can add an object to ymolchi because it's sort of reflexive - perhaps you can "wash yourself your hands", but not "wash your potatoes" as they aren't part of yourself.
Diolch am eich cygnor. I hope that is OK Welsh sentence. Thanks for your advise.