I had the same issue, no matter what I did it would not budge, Until................I put it in the freezer for about 20 minutes. Took it out and it turned easily
I just removed mine. It didn't want to come off, but i just grabbed part of a thin power cord for something that was attached to my computer, wrapped it gently around the lens cap and put very slight pressure on it. Came right off. No big deal.
I had tried numerous times with my fingers - no go.
I replaced it with a ND4 for the next time I get to fly. Too cold and windy these days...