I was under the impression that the て form followed by ん was a contraction of ～てるの. However, in this sentence we have a のか following it. The のか sounds like it's supposed to nominalize and question the previous phrase, though I'm not sure how that works, even having read a few past posts on the subject.
My attempt at translation is:
You're growing it out? Your hair?
I don't see any pondering going on there, though, so I'm not confident.
Seems basically the same to me at this point, which may help explain my confusion.