If it is threaded, it getting stuck can be a problem sometimes. What usually works for me, is to take it on to something like a tiled floor, and.. well.. drop it from about 3 inches up. Doesn't take enough to damage the mod.. just rattle it. I have a Nemesis that gets stuck all the time, and that does it for me better than pliers or a vice.
A second thing I use, but I doubt you have it on hand, is a
rubber strap wrench It applies pressure to all the sides at once, and doesn't leave a mark. If you had one of those, that would be my first choice. If not, wrap a rubberband around the top, and use pliers. Try not to scratch it, or use a vice. I am afraid you'll deform the metal tube and make it pretty much a permanent thing.