Can Moonfall select a unit already at a battlefield where your unit is, or no since it's not moving it? Does it still -2 the units?

No and yes.

You cannot select a unit already at the chosen battlefield for Moonfall's move effect. Moving is defined as a permanent changing from one board location to another (Rules 420.1, 440.1). A unit already at that battlefield has nowhere to "move to" — you can't move it to where it already stands. You may choose zero units to move instead.

However, the -2 Might still applies to all enemy units currently at that battlefield, regardless of whether you moved anyone. The second sentence ("Then give enemy units there -2 this turn") is separate from the move instruction and affects whatever enemy units are present at that moment.