Doing some reading today and it seems like this may be related to diatoms.
I've also got some dark streaks/bands of water in the pond so the two may be coupled as I only saw the dark bands of water a couple of weeks ago - around the same time I added the pump to move more water before I decided to do the aeration system.
Here's one of the dark streaks which is more visible after adding some pond dye.
I'll get some picture of the stuff attached to the pump tonight.
