I am going to tell you what I know. I have two ducks for the same reason. I have owned them for two months.
Being in a new environment, the ducks may not want the mash anymore. Switch to pellets.
As for the poop, I know a chicken poops 22 times a day. I haven't seen any stats on ducks, but it seems they aren't pooping machines.
The mud is a problem because of parasites. Keep you eyes open for any issues with worms.
If you can, buy a ton or five (depending on your duck run) of stones. It will allow water to drain, and keep the ground from turning to soup. I must say that so far I don't have mud in there, but that may be because they aren't swimming in the cold.