There is no need for a hyphen in this adjective, for the following reasons.
Adjectives of the form adverb + adjective (eg, "slow-moving", "fully grown") need a hyphen only if the adverb is also an adjective that could be taken to apply to the following noun. Hence "a fully grown man" needs no hyphen (because "fully" is not an adjective, and so this can't be interpreted as "a man that is fully and grown") but "a slow-moving plot" does (because "slow" is a adjective that can also apply to "plot": "a slow moving plot" means "a plot that is slow and moving", which has a different meaning from "a plot that is slow-moving").
As "once" is an adverb and not an adjective; and also because "once removed" is only used after a noun, "once removed" needs no adjective.
That said, looking through the first few pages of results for a Google search on "once removed", I see 28 without a hyphen and 2 with. I propose therefore to move this page to "once removed" and have "once-removed" as the alternative form.
— Paul G 08:58, 30 Jun 2004 (UTC)