Common problem, but not a big problem.
Try this. In between casting sessions, put the pot in a cardboard box that is just large enough to hold it, and put about half a dozen to a dozen mothballs in the pot.
As the mothballs sublimate, they drive away moisture laden air.
If that doesn't work, the area where you keep it is just too damp, so you'll need to try something with a better seal, such as a Rubbermail container or a very large (20 or 30mm) ammo can.
Beware the man with the S&W .357 Mag.
Chances are he knows how to use it.