Used to figure moldy cardboard was just part of the deal with seed starting. Turns out air flow matters more than I thought, and stacking them tight like that traps all the wet.

14 months on plastic, was it left out in the sun?

Curious what your setup was when you got burned. Were you shooting the laser from the bottom landing up the whole run, or were you moving it and re-leveling partway? And did you check it against a level or plumb line first, or just trust the bubble right out of the case? I ask because lasers drift on you, especially if the tripod gets bumped or the thing sits in the sun for a bit. Dale's string line trick works because it doesn't care about any of that.