If your guitar is electric then it won’t work properly into a line input. The mixer has no hi-z in as far as i can see so really you need a d.i box or some other form of direct recording unit between guitar and mixer.
Your piano should be ok.
If you unplug the instrument and record blank audio does it have the noise?
Have you tried another input?
Have you tried a different cable
Are you using the correct cable - jack plug both ends?
The only thing other than piano I can hear is a marimba type sound. I’m guessing that isn’t the mysterious noise
So if you hear noise when playing back it must be caused by something on playback…is it in Cubase? Are there any plugins in the project? Do you hear the noise if you play it in another media player? Do you hear any noise of you play audio from somewhere else?
I still don’t hear a lot of noise other than the guitar (it would be easier to id the artifacts without a noise gate).
It is recorded at a very low level which is presumably because you’re feeding a guitar into a line input when really you need a DI box (as Grim already stated). I do wonder if what you’re hearing is feedback as you’re monitoring through the mixer which as far as i can see puts everything back to the input?
The keyboard sounds fine, the guitar has a lot of hiss and a gate that makes it more noticeable.
Recording at -30dB might be a little on the safe side.
As others have said, a DI box also called a line-driver, would be a good and cheap way to get better sound.
There a active ones, that need a battery or Phantom power, and there a passive ones that uses a tranny.
A cheap passive DI colors the tone more than cheap active DI, good transformers are not cheap, the great advantage of passive DI’s is that you can send a recorded line-signal into the Output of the DI and get a guitar signal out the Input of the DI. Sort of in reverse setup, that signal is then perfect for a guitar amp, as it has the right impedance and signal strength.