The mold is caused by your soil - it is a common problem when mixes containing soil are used to germinate. I suggest you use instead a so-called sterile soiless mix - Jiffy Mix is a common brand and readily available. If you can't find it any other soiless mix will work. Some make their own from equal parts of peat and vermiculite or perlite but home sterilization can be difficult.

Oh. Okay. Thank you for the reply! I need to double check if it's safe for guinea pigs. I've done a preliminary search that claims perlite is non-toxic? Do you happen to know if these things are non-toxic or note? The problem I've had with potting soils, is they all contain plant food. So, I've been leery of them.

Do you happen to know how to sterilize dirt?

Thanks!