This usually will come off when using a powder type cleaner like Bar Keepers Friend and a green scrubby.

Mine gets something like this. In my case it's mineral deposits from hard water. Bar Keeper's Friend is acidic and should work well (although I've never used it). I usually use white vinegar. I set the crockpot in various positions so that a small amount of vinegar will sit in one area at a time and dissolve the mineral deposits. Usually the worst seems to be the line at the top of wherever the liquid level was in the last recipe. Bar Keeper's might make a nice paste (when mixed w/ some water) to make it sit in that area for a few minutes...think I'll try that next time.