brown sugar has gained some moisture, want to remove
Refrigerating things usually dries stuff out in modern refrigerators, which tend to remove moisture, rather than retain them.  I agree with sourd'oh that drying is likely your issue.

It sounds like you are asking how to remove clumps.  I use a sifter, or try to move the grains through a strainer with a fork.  What are you using the brown sugar for?  Will it dissolve during mixing anyway, removing clumps (as in when beating cake batter)?