I think I have a large clump of mucus is stuck in my nose and I can't get it out. I can see the clump moving freely way in the back of my nostril.?

Likely a polyp. If you can't blow the mucus out and you can see it... It's probably a large polyp... Best to see your local ent.
Sinus Rinse. If you can't blow it out, consider going to any pharmacy and picking up a neil-med sinus rinse kit. If it's really not a polyp or some other soft-tissue mass, then i'd expect you could rinse it out with the saline solution. Or, you could use an infant bulb-suction device to suck it out.