When a material, different than the upper, is used as the vamp on a moc-toe style shoe the inserted material is referred to as a “Plug”.
When the upper is made of leather, and the plug is made of fabric (canvas, linen, etc…), that just becomes a pain to polish.
Trying to keep the polish color off of the fabric, while brushing the shoe, after a good coat of polish has been applied is almost impossible.
Some people try to cover the fabric with tape. Some rub the polish on with a cloth and never brush. Some only use neutral polish. I have a different approach altogether.
I simply polish the shoe like I would any other, without applying polish directly on the fabric of course. As I brush the shoe some slight amount of polish will end up on the fabric, but I don’t care.
Once I have polished the upper to the level of shine that I desire. I get a small ramekin of warm water and mix in about a quarter of a teaspoon of Oxiclean (which you can pick up almost anywhere you find cloths soap). I take a medium to soft toothbrush and move it quickly back and forth in the mixture to create a slight foam.
I then take the toothbrush and lightly scrub the entire fabric plug (so as not to leave any water lines). Some of the cleaner will get on the leather, but I just wipe it off with a damp cloth or cotton round.
If you end up with any white residue on the fabric, just scrub the plug again with clean water to remove the excess cleaner (remember to do the entire plug).