I keep seeing a speck in my field of view and moves to the side then its gone, it's only in my left eye. what can cause this?

Floater: This is a membrane fragment in the vitreous gel which fills the back of the eye. It moves with eye motion and if close to the center will drift in and out of view. These are benign and only a problem if accompanied by lowering of vision, light flashes, or the rapid appearance of dozens of floaters.

It could be a floate: It could be a floater. Do you have eye pain, light sensitivity? See your ophthamologist and let them check you.