Is this a window a/c unit? All a/c units condense water out of the air, the quantity depends on the unit and the relative humidity. Unless there is a drain in the wall, it's probably running outside, maybe down the wall. If it IS draining into the wall, you've got big problems. If the unit has not been cleaned, the drain could be plugged, and the drain pan is overflowing and not going out the drain pipe. Looks like theres some maintenance needed, or maybe a hose line or extension to the drain to direct it away from the wall outside.
If it is a central air unit, the system's drain may run down that wall, and there's probably not much you can do about it unless you move it.