When you say the moss I'm right in thinking you mean what I'd refer to as the Lancashire mosses - Tarleton, Banks, Burscough etc?
The potatoes are being sprayed off, probably with glyphosate, to aid or ease harvesting. Two points here. First killing off the top growth simply clears the soil so machinery can easily pass over. The second could be to control growth for one of two reasons - the tubers have reached the required size or there is no market. Either way the tubers will stop developing/increasing in size.
The tubers are unaffected because glyphosate is a contact herbicide. There are five types of herbicide, contact, residual, selective, non-selective and systemic. Contact herbicides basically kill whatever they land on. Systemic herbicides are absorbed by the plant and attack all areas from within. Residual sit in the soil for months. While selective and non-selective either target specific plant types - broadleaf weeds for example - or kill everything in sight!!
There is evidence glyphosate used in this manner is allowing it to enter the food chain.
