Question

What are the possible spin quantum numbers for an electron with `m_l = -1`?

Answer 1

Whichever you want: `-1/2` or `+1/2`. There was no `m_s` given, which specifies the spin of the electron.

Note that by specifying `m_l = -1`, you haven't explicitly shown what `l` is. Since `m_l = {-l, -l+1, . . . , 0, . . . , l - 1, l}`, `m_l = -1` could correspond to any `l >= 1`.

Either way, we do have some sort of orbital with `m_l = -1`, whatever it may be. So, the electron has no particular spin other than the two possibilities of spin up or spin down.