That looks like Toxic Salts build up to me.

You over feed them with the wrong nutrients at the wrong cycle and the Ph is off so even if the nutrients are there the plant can't up take them or use them.

You need to flush 100%...watch your Ph and when you do feed them back with nutrients you need to start back at 1/2 strength one time then keep up with the cycle if life feeding for their age and what ever cycle they are in at that time.

With out more information that is just a guess....I bet I'm right about my guess though.