soo, it was too wet and cause the mold. you've to get rid of the water. The media need to be moist [ not wet or standing on water ]. It's fine to use cover to get the seeds to germinate, and not alot of water is needed to moisten the media, since water is recycle within by evaporation and condensation.
you will be opening the cover to check the progress regularly but make sure the media don't dry out.
see what you've done, you've some work to do now
. pick out the affected seeds and clean them by placing in small container with water, add a drop of washing detergent and give a good shake. once done place seeds in another container with sterile water, swirl it to clean excess impurities from the seeds. Alternatively use fungicide.