A theory of predication invokes immanence if it explains why snow is white by introducing something that is in snow that accounts for its being white. Aristotle's theory of predication in the Categories is partly immanentist, see Cat. 2, 1a24-25. My object here is to shed some indirect light on this passage. I suggest that the comment is a disclaimer responding to an immanentist theory of predication under discussion in the Academy, according to which the something that is immanent in snow that makes it white is a physical ingredient. This theory was an idea of Eudoxus'. Aristotle was sympathetic to the position, and his own sounded a lot like it, but his was not that position, and so it was important to distance himself from it.
Dancy, Russell M., "Predication and Immanence: Anaxagoras, Plato, Eudoxus, and Aristotle" (1984). The Society for Ancient Greek Philosophy Newsletter. 118.