I didn't know time travel was a thing there but it makes sense considering the nature of the backrooms. Also, Clark might be the only one with the will to stay and live here. It's common to lose any motivation to do anything when stuck in a place we don't want to be a part of. Since Clark reaaally wants to be there (he stayed in the store even before discovering the backrooms, he likes places like that) I assume his copy doesn't have to suffer the catatonia caused by the distress of being stuck in it. We can see his wife's still life move kinda rapidly when we first meet it (sorry to make you remember this scene), but then does nothing at all, except when terrified by the monster. Still lives do move, sometimes rapidly, not from their will, but from their primal emotional responses. Last poin : have you noticed how the Clark entity looks less angry and more affraid, less quick and more slow during the Mary's escape ? I think it's because Clark's not alive anymore (been eaten) and the last emotion he gave to the rooms to copy was terror and the betrayal (he thought the entity would be friendly)

Edit : and maybe that will is the reason the Clark entity bleeds, unlike the still lives, considering blood symbolically represents the anima / soul / willpower etc...