koralias are notorious for the impellers going backwards or in reverse when restarted or used in an on/off wave maker mode.
Suggest you turn it on and off until it flows in the right direction or you can be impatient like me and stick something into the prop to stop it then let it spin to see if it corrects itself, might take a few times this way also. Try the on/off first if your afraid the other way may break something.