Hi pdpond,
I'm going to chime in even though I'm no fish expert. My only experience with ich was quite a few years ago with some aquarium fish that I had. It started out as little clear "bubbles" adhering to the fish. The "bubbles" eventually turned white like little tiny cotton balls stuck to the fish. If I remember correctly, ich is a fungal infection rather than a bacterial / viral one.
I put my pond in three years ago loaded with goldfish and some common brown minnows, and so far ( fingers crossed ) they're all healthy and thriving, even giving some offspring. I'm wondering about that 6.4 pH reading you got. I've been told to measure pH several times a day as the pH can vary though the course of a day. My pH seems to hover between 8.0 and 8.4, but again from my understanding, a big factor is big pH swings. You might want to look into that.
Someone suggested that you take your fish to a vet or an aquatics / fish shop to see if they can say what it is. It's a good idea. If it is ich, there's an anti-fungal medication you can get to add to the water, however I've only seen indoor aquarium sizes for that medication. As for the batting that don't ask suggested; it's the synthetic white "pillow filling" I tried using that in various ways when I first started with my pond. If you have a big algae problem, that stuff works well to start, but clogs up really fast. My pond is only 600 to 800 gallons, and I don't know if this works with single cell algae, but I use a single 250 ml bottle of 3% hydrogen peroxide once every two weeks to keep string algae at bay. Others have differing opinions on this, but it seems to work well for me.
Good luck and I hope you get your problem solved.
Paul