Vinegar cleans vases you might try that. Let it soak for awhile.

Have you tried the commercial products like Lime Away? They are not designed for use with cooking appliances, so you would have to make sure you rinse the pot really well afterwards.

With vinegar, as Emma has said, the trick is to soak the item for hours. You can't just spray it on and wipe off.

I would try CLR. That seems to work with about anything and is safer to use than LimeAway. You may have to let it soak for an extended period of time and definitely be sure to rinse it really well. I have also used a combination of lemon juice and vinegar. You might try heating it, mixed with water, in the microwave first, and then pouring it in and covering the crockpot, over a low heat. Good luck. I hope one of these solutions works for you.

next time use distilled water, and clean thoroughly between uses

Fill crock pot full of white vinegar and let soak for a few days. Do not add water.

Be sure and clean crock pot daily when using as a humidifier. Wet inside of crock pot with vinegar, let set couple hours to remove a days worth of lime. Re-wet off and on during that time.

Sometimes cleaning crock pot with baking soda will remove small amounts of lime residue before it becomes scale. Other kitchen powder cleansers will do the same. Use vinegar for scale.

Lemishine - it's safe (just citric acid like from lemons) sold for use in dishwashers. It works great for coffee pots etc.

each time my electric kettle, humidifier, etc gets caked up with lime scale I clean it with vinegar. It is MAGIC. For the kettle, and humidifier, I use half vinegar and half water, fill to desired level and turn the appliance on. ( this makes vinegar fumes but I don't mind). After the mixture gets good and hot, I turn it off and let it sit to do its work. For my kettle and humidifier, which heat up fast, this is a 5 minute process. After which I actually save the vinegar/water mixture to use again cause I am a cheap skate and I get several uses out of it- but I don't dilute it again. I do this on a weekly basis in the winter and my appliances are like new. I hope it works for you, too.

i vaguely remember using cream of tartar to clean the hard water crust out of my baby bottle sterilizer. I don't remember the recipe but some was added to plain water, heated up and I can't recall if it was left to boil for a while or turned off and let set.

Vinegar is your cheapest bet....but you said it doesn't touch the build up.
Why do you need it to be gone?just continut until the crock pot gvies up.
But for the future, a dehumidifier puts out distilled water and if you have one or know someone who does, you can save that water in gallon jugs for winter use.