A word or phrase for a fake doctors or healers

If you are talking medicine (including pseudo-scientific medicine) then you want either quack or snake-oil salesman.

Quack (MW, noun definitions 1 & 2)

charlatan

a pretender to medical skill

Snake-oil (MW)

any of various substances or mixtures sold (as by a traveling medicine show) as medicine usually without regard to their medical worth or properties

Snake-oil salesman (Cambridge)

someone who deceives people in order to get money from them

"Quack" is the most common term I know of. Occasionally, it can be used as a derogatory term for any medical doctor

If you want something a bit more verbose/proper, use the original term "Quacksalver"

From Wiktionary:

c 1570, from Dutch kwakzalver ‎(“hawker of salve”), from Middle Dutch quacken ‎(“to brag, boast; to croak”)

i.e. Someone who boasts about the potency of their remedies, but is in fact a fraud.