I have seen that happen to Wynne every once in a while during some of my playthroughs. I'm not quite sure, but it might be because of death syphon. Could you please tell which sustainables Morrigan is set to use during combat?
This happens to all my mages occasionally. It's an FX from the Arcane Bolt spell that sometimes sticks around after the battle. Saving and reloading should get rid of it.