No it is not safe.
A 15 ampere rated receptacle should not be installed on a circuit protected by a 30 ampere breaker.
You should not be plugging devices that draw more than 15 amperes, into a 15 ampere rated receptacle.
The receptacle in your photo is a 20 ampere receptacle, so you shouldn't have a problem plugging in a 20 ampere device. They make receptacle configurations different, so that you can't plug things into the wrong receptacle. By making a bootleg extension cord, you're bypassing that safety feature.
You're going to burn the place down.
Also note that since the device is active for longer than 3 hours, it would be considered a continuous load. In which case the circuit (overcurrent protection, wiring, and devices) should all be rated for 125% of the load. That would mean the wiring, overcurrent device, and receptacle should be rated to handle at least 22.5 amperes (18 × 1.25). That means a 30 ampere breaker, 10 AWG copper conductors, and a 30 ampere receptacle.
Your AC only draws 6.3amps when its running. But when it turns on, for a moment as the motors start, it draws significantly more. Many window air conditioners say in the manual that they need a dedicated circuit because of this.
These startup surges may melt cheap powerbars and splitters, eventually causing enough damage to be a fire hazard.
Additionally, those surges cause the voltage to drop which can wreak havoc on delicate electronics like an electric piano. Usually not destructive, but enough to make it malfunction.
Assuming you cant get permission to have more outlets installed, run an extension cord, preferably for the other electronics near the window. Make sure that extension runs to an outlet on another circuit, and that no other large loads or sensitive electronics are on that circuit.
Best Answer
Most AC units say specifically NOT to use extension cords. The reason for overheating is voltage drop caused by the possible smaller gauge of wire and length of the extension cord. An appliance rated at 12amps probably has a 14 or 12 gauge cord. If your extension is a smaller gauge, it may be too small for the current and heating up like an old fashion fuse.