Elixir does not have multiline comments.
However, one very common use case for multiline comments is documenting modules and functions, for which you can use the module attributes @doc and @moduledoc together with heredocs.
defmodule MyModule do
@moduledoc """
This module is great at X
"""
@doc """
Frobnicates the given string.
"""
def frobnicate(s) do
end
end