Hi, ElixirLS does not currently support syntax highlighting errors in .exs files:
https://github.com/elixir-lsp/elixir-ls/#known-issueslimitations
Here is the tracking issue for this:
https://github.com/elixir-lsp/elixir-ls/issues/89
Also this may become less important in the future if ex_unit begins to support compiled test files:
https://github.com/elixir-lang/elixir/issues/10983
