You're sure it's not squirty cecal poo hitting the wall? You should be able to tell the difference, cecal poo on the wall will look like something splashed there on its way down, with well-defined edges, whereas a mildewy patch is usually not oriented w/r/t gravity and has more-indistinct edges. Also poo will be brownish (sometimes very chocolate-dark-brown) whereas mold or mildew rarely is that color.
If it *is* mold/mildew, is this above or below the level of the droppings board? If above, I suppose it could be from localized dampness given off by poo piling up on the droppings board, solution being to scrape the board off more often (like maybe daily), which if you set things up right takes literally only seconds. If it is *below* the level of the droppings board I would tend to doubt the droppings board has much to do with it, unless some fraction of poo is missing the droppings board and piling up on the bedding below, in which case see previous sentence about removing poo deposits more often.
Other factors that can produce patches of mold/mildew on interior coop walls include moisture leaking thru from roof or siding or window flashing; a defect in insulation (if the rest of the wall is insulated) such as damp or missing insulation in that area so it creates a cold spot on the wall; or airflow patterns that cause air not to circulate as well in that locations (especially if your droppings board is low so this is a rather small confined space under it)
So, hopefully maybe you can look at those suggestions and find something that rings a bell?
Good luck, have fun,
Pat
