Hello, here an answer from a small town in the neighbourhood of Gouda, although that doesn't make me an expert on Gouda cheese
It's fairly normal to get some mold on an unwaxed or uncoated cheese. You did well to wash it with brine and I suggest you do it again a couple of hours before you wax it, so it has time to dry. I'm cleaning my cave and my coated Gouda's from time to time with some water with vinegar.