ive had plecos with discus for a long time
with no issues what so ever
but on other forums ive always read about thier plecos or siamensis or flying fox or cae chasing thier discus all over the tank
so its a chance your taking
i would assume at lights out an your discus are sleeping, the algea eaters
are on the prowl for any flat surface with algea and come upon your flat discus thinking its just another spot to eat.
then they snack on some discus slime coat and are on it like white on rice