Welcome to the forum.
Look at the size and health of your fish! koi are not the most durable od fish and are prone to have issues when water quality suffers.
So even though the ponds may not be spotless and have some build up i would do any changes GRADUALY. The muck is just that to us but in the nature world it is home to all sorts of creatures bacteria and microbes.
What type of filters do you have and pumps.
There are many a visitor to this site that would kill to have conditions as you have now as a final product. Net out the leaves , it appears you have some string algae but looks minimal just pull that out without squeezing it as when you do you will release spores into the water.
It appears you have a great start to your new hobby i would caution about any fast and drastic changes if you have two ponds the water flows through both then i'd look at doing a light cleaning to one, and see how the system does. and by light i'm with the masses here vac out the leaves and other material that can rot and cause issues . no pressure washers .
your wanting to feed the fish will probably the biggest downfall to the water quality
to replace those fish won't be cheap for that size and quality minimum of 800 for the gin rin kohaku