Here is a picture of the red spots on my dragon fruit. I believe it is just the little tendrils that leave a mark after they have dried up, but the white crusting is NOT normal.
You say you are watering every 3-4 days. Even though dragon fruit cacti is tropical, it still is a cactus. You are watering way too much! You only need to water about once every week or even 2 weeks! You can tell when you insert your finger about 4 inches deep in the soil, and it feels dry. Then, you can water.
Can you move a little of the soil to expose the base? Do you see the flesh rotting? Dragon fruits are prone to stem rot when over watered.