I'm wondering about that little appendage that’s coming out, what’s that and what’s it doing? Seems to be going away from the light.
I tried to google but most result for my searches were just offers to buy a new cactus
Looks like it is showing some etoiliation on the new bud. It has to bud to continue to grow as it was cut of when planted. Aerial root may mean that it is root bound in that pot.
Larger pot with the correct soil for dragon fruit, more sun.