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Looking at the syntax file, it looks like the highlight won't trigger for # is you set let g:hs_allow_hash_operator=1 (https://github.com/neovim/neovim/blob/dc8273f2f1e615cac5cee86709ca4ae6dbd70edf/runtime/syntax/haskell.vim#L113-L115)
That said, I find that the builtin syntax files for Haskell are really inadequate. For a better experience, there is https://github.com/neovimhaskell/haskell-vim. It's worth it just for its syntax file alone (so if you don't feel like installing the whole plugin, you could just copy syntax/haskell.vim to your config). It doesn't have an issue with #foo.