No, since a 5 volt pin on the Mega can only give you a max of 50ma (40 times less that what you need). You can get a really or something and have the humidifier be powered by a different power source. The Megan’s output of 5V can trigger the relay or controller board to power up the device.
In short, it is possible but only with a circuit and a different power source for the device. You can not directly connect the device to the 5V and GND since the Mega can not give that much current