Welcome to the forum Inky. A thick white film sounds like either a fungus in which salt will help, so will upping your water changes to 50% or more a day. Or it could be the shedding of their slime coat as a result of some type of irritant. When a foreign chemical is added to the water column, often times discus will shed their slime coat. For example many medications cause discus to shed their slime coat, other times a ph or nitrate burn will cause them to shed their slime coats.
Can you get any pictures of your fish? Can you prepare to do a salt bath for the affected fish should we determine its fungal? A good medication to have on hand for fungal breakouts is quick cure. Its a malachite green + formalin medication, it is strong so don't go dumping it into your tank just yet. Lets figure out what you've got going on first, pictures will help us a lot.,