This tiny creature is a Flatworm - Amphiscolops sp. This type is virtually clear, and quite small. The rounded end is inflated like a sail, and stretches in the direction it wants to travel, then like a suction cup it grabs the glass and pulls the rest of its body that direction. This is a benign type and you shouldn't worry if you see any in your tank. I've seen them on and off in reef tanks as well as refugiums, usually short-lived and forgotten later. Reef-safe.