Something akin to the following, which is neither tested nor I’m sure it works as posted:

dynamic = 
	value 
	|> String.split(" ")
	|> Enum.map(fn term ->
		term = String.replace(term, ~r/['?\\:‘’]/, " ")
		dynamic([], fragment("to_tsquery('simple', ' ? ':*')", ^term))
	end)
	|> Enum.reduce(fn dynamic, acc ->
		dynamic([], ^acc and ^dynamic)
	end

complete = dynamic([thing], fragment("? @@ ?", ^dynamic, thing.fulltext_index))

query = from(thing in Thing, where: ^complete)

Based on that message, it sounds like you tried code with a line like where: Enum.reduce(...). Building the fragment outside the Ecto macro plumbing may make things clearer.

Posting the code would help sort this out.

Thank you, this helped!
Here’s a slightly adjusted version I ended up with, for posterity:

sql_joined_query_terms =
  value
  |> String.split(" ")
  |> Enum.map(fn term ->
    term = String.replace(term, ~r/['?\\:‘’]/, " ")
    dynamic([], fragment("to_tsquery('simple', ''' ' || ? || ' '':*')", ^term))
  end)
  |> Enum.reduce(fn sql_query_term, acc ->
    dynamic([], fragment("? && ?", ^acc, ^sql_query_term))
  end)

sql_query_conditions =
  dynamic([thing], fragment("? @@ ?", ^sql_joined_query_terms, thing.fulltext_index))

from(Thing, where: ^sql_query_conditions)