Please check if the "WIRELESS" mode is selected on the Bluetooth devices or not. (If the "USB" mode is selected, it can't be connected by Bluetooth.)
Please check if the Bluetooth devices is connected to another device or not. When the power LED stays on in blue (i.e. it's not blinking in blue), then it cannot be used since it's already connected to another device.
Maybe the Bluetooth MIDI Connect app is required? Also, when are you coming out with an Android version?
I can't find the Microkey air 2 on the Bluetooth devices list on my iPad pro 9.7.