Almost all of the Alternanthera in my tank has these long white things growing out of them at their nodes. Are these roots or are they going to be new leaves?
No, they're aerial roots. Lots of stem plants produce them.
jinkz is correct. When a plant cannot get a stable amount of nutrients from the substrate or even if the nutrient level in the water is low, it will grow more roots all along the stem searching for what it can get
btw this might point to nutrient deficiency but it also shows that your plant is trying to grow and is converting to its submerged growth
i think that the scchool of thought is that they produce those roots when the plants are searching for nutrients they are lacking