I would have skipped the plastic and just laid gray paper under the laminate. Be prepared for it to take forever to get it even and without bounce, depending on how meticulous you are. The moisture pressure downward is very small, and on the contrary, it looks like this mold has grown because moisture has settled there without being able to dry out—perhaps because there's been too tight a floor above? Was there a plastic mat at some point before, maybe?
It's important to add a little heat so the house can dry out. In an old house, you shouldn't put in plastic, it will just turn out poorly. Stick to diffusion-open materials and it will be fine.