I fucked this poor little guy up when I first got him. Set him in my green house and forgot to open vent and he fried. been trying to save him and 2 little leaves grew and im pretty sure they dried up. Repotted in 2 parts cactus soil, one part compost, one part forest soil and brought him inside and put a ziplock bag over him for humidity and vent it like every 3-4 hours. Any advice as what this white fuzzy shit is? Looks kinda like mildew but I don't really think so. Any advice?

B.Caapi can rot very easily. You need to make sure to ventilate it.
Try to carefully rub off the mold and change the soil its in. When the mold turned green/blue or any other color you have spores everywhere and need to clean your greenhouse.

What im doing with my peyote to keep him from molding is that I water him and then dry the upper layer of soil and the cacti with paper towels and a blowdryer. This really keeps all the fungus away.
You could treat the caapi with alcohol to kill the fungi but I dont know how much this will hurt the vine.
Also a way to get rid of mold is direct sunlight. Most mold does not tolerate uv very well.