Plant Health: Brown spots and covering

My first grow is going on. Till now everything was just fine. A 1.5 week ago i noticed that one of 4 plants started to render brown spots and tips begin to turn lime-yellow which soon progressed to same brownish crap. So it starts from tips firstly. I used back then liquid fertilizer (10/4/6 NPK) but it didn't help much. Today, other neighbour plants started to show brown spots. What's up with it and what to do?

Thanks

looks like some nutes splashed on the leaf when you fed them