Its a fungus with all the dampness this year I've had this too just mix a weak bleach solution 20 to 1 water to bleach and mist the affected area do it at mid day as bleach will dissipate in sun light or the chlorine will , the white area will turn dark in a day or so and then you clean it up and put it in the compost pile . A shop Vac. will also get rid of it. You could also just leave it . It might get bigger but shouldn't hurt any other plants it's just a little unsightly.