Yup, that's a tree. A huge tree, apparently going through all floors and occupying the entire building, while having managed to keep far enough away from the tinted windows that it wasn't visible from the outside. The tree moves, sluggishly, but still in ways trees ought not move.

Its purpose becomes clear, as well: in addition to the several differently-mutated monkeys here and there, many of which are fighting Purity and Crusader's clones (even being impaled is not enough to stop them most of the time), there are several monkeys connected to the tree, duplicating quickly, with mutations, and then having one of the duplicates killed for nutrients, with some help from other monkeys to ensure only "good" mutations persist.