Can you say cleaning?
No, I don't think so.
(transitive) To remove dirt from a place or object.
Can you clean the windows today?
(transitive) To tidy up, make a place neat.
Clean your room right now!
Not for a person, though.
You can use 'clean' to refer to a person washing up, too, but it implies washing a specific body part, washing up plus other general grooming (e.g. brushing hair, etc.), or sometimes if it's excessive washing due to some psychological need.
Shouldn't "at the moment" also be a viable translation for "teraz"?
Yes, that makes sense. Added.
The boy washes himself now is wrong?
Well, if it's "now", then it's "is washing" and not "washes", right?
without any other context i presumed it meant his mother no longer
has to wash him
That would mean "myje się sam" or "myje się samemu" (sam/samemu = by himself).
How about "The boy is washing up now"?
I asked about it and got an answer that in AmE it is "only associated with toilet usage, hands, and maybe face", and in BrE it means washing the dishes...
How about: "Now the boy is washing himself."?
OK, let's add it.