The hairiness of the cakes in the picture are the mycelium fighting off something. It's nothing to worry about because the fully colonized cake is strong enough to defend itself and as long as the hair is white you're good. Green hair is bad, but white is OK. After the cakes last flush and they are done, you can tell they are dying because they will then get really hairy and sometimes turn green. At the first sign of that, toss them.
And, yeah, I'd put a piece of aluminium foil under each cake to keep the perlite from sticking to it.