I just rinse mine under running water. On a related note, if you end up with a buildup of shellac on your MM, if you can't clean it with soap and water, you may be out of luck.
It's shellac, so clean it with denatured alcohol, right? Nope. Denatured alcohol and MicroMesh do NOT get along. I did not conduct extensive tests, but my limited experience wet sanding with DNA had the abrasive coming off the MM pad. I say limited, because I stopped trying it at Green. Something seemed a little odd with the first MM pad, but because the pad was pretty much the same color as the wood, I didn't see what was happening. When the greed MM started showing up on the wood, I knew things were going sideways.