Hi Richard, welcome to Yamaha Synth!

Everything is perfectly normal...

Like all Yamaha synthesizers, the MOXF outputs signals via MIDI at all times. Not all of them are associated with you touching keys or moving controllers. The MIDI ACTIVITY indicator on the Transport bar will continue to show activity because it is properly seeing this activity. The signals are likely "F8" and "FE".

"F8" is Midi Clock - the MOXF has its own clock. If you have the MOXF set to send MIDI CLOCK a stream of clock messages will be detected. MIDI clock is always running unless you turn CLOCK OUT = OFF. When you are connected to a DAW (like Cubase) it most likely will be your master clock, so the MOXF Clock will likely be following clock messages of Cubase. You can turn Clock Out = OFF
Press [UTILITY]
Press [F6] MIDI
Press [SF3] SYNC
Set CLOCK OUT = OFF

"FE" is Active Sensing - all Midi devices when connected are supposed to identify themselves, every couple of hundred milliseconds an "FE" message is sent. This way, if disconnected or a break in connection appears, the system will identify which device is having the problem. (Many companies do not send FE, but Yamaha being a founding member of Midi, still does follow the basic rules). Active Sensing is always sending data...

If you would rather not see the MIDI ACTIVITY of your connected devices on your Transport bar you can opt to not view it. The items that appear on your Cubase Transport are customizable. Here's how:
Right click on the Transport (exactly where the Midi Activity indicator is shown)
Uncheck the "MIDI ACTIVITY" option.

This will not affect the individual MIDI Track's MIDI ACTIVITY indicator, only the global MIDI ACTIVITY indicator on the Transport bar. This will become apart of your Cubase setup preferences, until you decide to change it.

Hope that helps.