Want to power an AP that requires 15.4 watts. Can I use this Mikrotik to accomplish this if that AP is the only thing plugged in and pulling power?

its rated to 1Amp max per port, 2 amp max total.
So if you power the Hex POE Lite with the included PSU that is 24V, your device will pull 15,4W/24W=0,64A which should be fine.
Be sure that the device you should power supports the Mikrotik passive POE way - devices that requires standard POE (48V, 802.11af) will not work.