Sorry about your problems but welcome to Koiphen. I can't see the white on the mouth but it is fairly common. It is called mouth rot and is a form of fungus, usually associate with a wound to the mouth. For fungus, it is best treated with Malachite Green, which can be used as a treatment for the pond.
Often this is associated with less than pristine water and we would be better able to assist if we knew the test results for ammonia, nitrite, nitrate, pH morning and evening, KH, pond volume, number of fish, age of system, and type and size of filtration. The test results should be from drop type test kits rather than dip strips as the dip strips tend to be very time and temperature sensitive making them less than accurate most of the time. New ponds and new filters lead to many of the problems that we try to assist with and with good information we can help to improve any deficiencies.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard