Using old promix for seed starting?

I have maybe a third of a bag of compressed promix left from last year. I haven't checked it recently, but I remember there was some white fungus growing on it during the summer. It's supposed to have mycorrhizal fungi added to it, and I read the growth I saw was probably harmless to adult plants. I had no trouble with the plants I planted in it.

I've heard promix is good for seed starting, so I thought about using it. However, that fungus has me concerned. If I use it, should I bake it to sterilize it? Won't that kill the mycorrhizal fungi? Is the fungal growth likely to harm seedlings? Or would it compete with the microbes that cause damping off?

Sorry if this seems like a "duh"; I just don't want to buy special seedling mix if I don't need to.