It mostly depends on how large the leftover patches are. The back isn't as worrisome as the neck area. But with the added moss if you keep it damp over the next few days he may just manage to take that last little bit off on his own.
If it's still a large patch, I would keep the humidity up for sure the next couple of days and try another soak after. Or letting him slither around in a wet pillowcase for a while before trying the washcloths again if you want to avoid the soak.
A few scales isn't too much of a worry, but a larger patch can lead to problems with the next shed or infections if the skin underneath gets irritated and the leftover shed packs the bacteria in.
It's mostly the neck area I would worry most on because the skin there isn't as tough and protected as the back.