I'm afraid the odd cat poo is a fact of life, Obviously it will prevent the plant nearest to it from growing properly, but ants tend to eat them pretty quickly, I am surprised you managed to have one old enough to go white as the ants are usually pretty quick off the mark. I managed to keep cats off my veg garden by covering each row with a length of wire mesh, which also keeps the pigeons off. Once the plants get big enough and cover all the soil, the cats stay away on the whole. The problem has been completely solved this year, by having raised beds. They haven't been inclined to use those at all. We have no cats, but one neighbour has three, the other has two and there are various others all looking for fresh territory. As I also have fox poo and hedgehog poo to contend with, to say nothing of goose and chicken poo and pigeon poo, the odd bit of cat poo in exchange for their disposing of my mice seems a small price to pay - except I do wish they wouldn't leave them right on my path to my outside freezer for me to tread on in the dark!