I suspect you might be dealing with an algal bloom rather than mildew. If it's truly mildew try a fungicide. Perhaps a bit less humidity could help.

Sorry for my English, In fact there is Algae growing with my seedlings, what can I do in such case?
I put away all the seedlings and planted in new soil, but Is there an algaecide safe for seedlings?
Alfonso

I don't think you will have a problem with this. It shouldn't overwhelm the seedlings, these are fairly robust. In a few weeks time you can start to let the surface dry out and the algae will stop growing.