Hmmmm! I have always assumed a fertile pond due to the muddiness and the fact that it has at least one obnoxiously green bloom every year and has had a couple floaty scum explosions this year.
Can nutrients come into the pond with a heavy rain flush (from the cattle pastures) that causes a wild bloom which sucks the nutrients back out in a few weeks?
Then the pond goes back to being muddy. Whether the pond is experiencing a bloom or is just muddy, both scenarios are discouraging and their does not seem to be a happy in-between.