The white stuff is likely fungus and will go away on its own. It is not harmful.
The plants are suffering from some sort of trauma at the moment. Be that too high temperature, too little light, or something killing them off like a chemical. If you have fish in the tank then it is unlikely to be a chemical, if the temperature is below 84F then it isn't that (unless it spiked up and wasn't noticed), and if the lighting is evenly spaced and bright enough then it probably isn't that unless they were damaged during shipping or kept in a dark place for too long before you planted them.
I am not familiar with your light fixture, can you provide details or a link on it?