Vinegar.......
White vinegar and a Dobie pad....the cruset is likely hard water salts...
And....I have removed the top rack and put crocks in my dishwasher...
Linda c

I also dishwasher my old crockery. Dobie pads are great..they're the plastic ones, aren't they? That is one way to tell age-related craze marks from those incorpoated into a design. In age incurred crazing, you will find that the darkness in the craze lines is much diminished after a trip through a dishwasher. That's why I questioned the footed dish in another thread. I wasn't questioning the estimate of age, only that the crazing didn't really strike me as age incurred. Not that I would put decorated pottery into a dishwasher. I have, but it's chancey as far as finishing is concerned.

Be careful. If the crock is glazed and the glaze is intact, any number of cleaning agent may be used. Vinegar (5% acetic acid) is good for disolving lime scale and other hard water mineral deposits. Ordinary salt is soluble in water, so I doubt that your white deposit is ordinary salt.

If the crock is porous or the glazed cracked, the pores and cracks may contain contaminants that may be harmful for foods.

A porous crock, or one with cracked glazing, should never be used for foods.

Fermenting crocks are new to me and I just bought one. I would like to clean it before I try sauerkraut. It's very old and looks pretty dirty inside and out.

The advice above holds..
I would add that "crocks" are vitrified stone ware....the ware actually fires together like porcelain and is not ever porous. if it is and has glaze cracks...it's not a stoneware crock.