White fungus on Roots

My seedlings were doing great and I transferred them into Happy Frog growth halted and dwindled down the plant. Pulled the plugs after 4 days of other stuff they have this white fuzz. What is it? What did I do wrong? What do I do to prevent this on my new seedlings?

Someone help

is that a spongespot? if so, i've heard of instances such as this. got to google and see what others have to say when they had this issue. gl.