Exposed copper wire

Chances are, what you see is the ground wire. It is NOT a current carrier (when installed correctly) and therefore is not insulated. It IS SAFE for it to be exposed.
If the wire you see is the hot or neutral wire, it should be covered with insulation up to the point where it is twisted together (with the fixture's hot or neutral), and the "twist" should be covered with a wire nut or similar insulated device.

Marc-First of all the fixture is mounted outdoors, this makes it a wet location and if sheathed electrical cable was used for the intallation this is wrong, minimum should have been UF cable.
Any exposed wiring can be a hazard, especially if it has a plastic sheath, it MUST BE PROTECTED.
You said only one screw is holding the fixture into place which means that the manufactures instructions were not followed.
Your question was can this be a fire-hazard, absolutely possible. Especially if the wiring treminations are exposed to the weather, wire-nutted connections have a tendency to corrode wheen exposed the elements corrosion causes high resistance points and this causes arching. Best to get it fixed correctly.
ttfn