What is wrong witth my mollies? I need your help?

All 3 of my mollies are acting odd, i have a silver,dalmation and copper molly.It started with one a couple of weeks ago,he was swimming strangely and darting around the tank,then yesterday i noticed the other two doing it and flicking themselves on things and today they have all started hanging around the surface and hiding in the corner of the tank.I also noticed today that one of them has a sort of white coating on his tail. They keep trembling as well(is this shimmy?)Im worried that what ever it is my other fish might catch it.The silver molly is a bully!!! Do u think it is stress or something more serious.My water quality is fine.i have a 65 ltr tank with 3 mollies,3 Cory's,6 cardinal tetra and 6 black neon tetra.Please help im desperate to do the right thing for my fish!!