White mould/fungi tendrils are normal and safe in mulch - don't be turned off! I'm guessing the areas under the plastic got warm and the fungi from the mushroom compost started the decomposition process. Simply turn the mould back into the soil. It won't harm your vege sedlings and will actually be great for all the healthy bacteria you want to grow in there. Good luck!
6/1/09 at 6:00am