Plate coral moving?

Everyday my plate coral is up against the glass. Part of it expands onto the glass when it is fully expanded. Also, every single day I move it away from the glass about 2-3 inches. When the lights come back on, it is back up against the glass. I didn't know these corals have the ability to move? Is the coral itself moving? Or is something in my tank moving things around.
 
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