It is a build-up of minerals from the water, mostly calcium and magnesium. If you water with distilled water for a while it will slowly dissolve. I wouldn't use any vinegar on it because that can screw up the soil pH.
The one stem on the right in the second picture might need to be cut and started over unless it's still putting out new growth. You could also ad a little potting soil to the top to cover the 'pinched' area and stimulate some new roots there. These are epiphytic plants and adapted to having new 'soil' added on top on a regular basis.
Otherwise nice looking cactus!
Happy plant parenting