Taking picture of the moon with cellphone through monocular?

I assume you're trying to take a picture like this one. I used a DSL in manual mode with a 300 mm lens and then cropped it. I used an ISO of 400, a shutter speed of 1/800 second, and stopped down the lenses to f/5.6. I also took other pictures with different shutter speeds to get the picture I wanted

I'm not sure what control you have on your cellphone, first you must be able to focus the monocular so that the Moon looks in focus on the cellphone screen. Its important to get the ISO higher than the default phone setting ( somewhere around 100) as this will require a slower (1/100 second) shutter speed and that would require a tripod. I am assuming the monocular will have a f stop fairly high like f 6 or so.

A longer exposure time is necessary

It will bring up the best detail

If your cellphone has a zoom up to x50 you should not need the eyepiece, just a steady stand

Typically a monocular increases the light gathering over the un aided eye.

This can result in so much light from the moon as to exceed the limit of the camera washing out the image.

My solution is like a cap over the objective, black matt ABS plastic, with a punched hole about half the diameter of the objective. Doesn't effect power or resolution, just cuts about 70% of the light.

So you know, those cheap Chinese camera monoculars are very poor to start with.

A tripod of sort really helps also.

We don't know what sort of 'phone you've got, but a different camera app might allow you to be more selective.  I think Open Camera allows spot metering, but it's been a while....

If you can, turn off the camera's autoexposure controls, and control them manually.

You need a faster shutter speed, and lower ISO.

The background sky is so dark, that the camera's autoexposure is reading that, instead of the Moon just by itself.