I hate to say this but you're keeping koi indoors. It's only natural that you're going to have algae spikes and will need to clean a TON. There's a reason people keep them in ponds
. It's the same with goldfish, oscars, and other large, messy species. It'd be a whole other story if you were keeping neon tetras or cherry barbs.
Like I said, the green water problem won't be solved by any fish. You could try live plants but you'll have to find some that your fish won't eat. As for the coating, yeah, a bristlenose pleco (yup, a bristlenose catfish is actually a pleco) might help depending on what kind of algae it is. But I wouldn't expect it to clean the whole tank and it'll depend on the individual fish.
Test your water parameters before bringing the pleco home and make sure you acclimate him slowly. You should also have hiding places so he can get away from the koi. Plecos can be fed blanched vegetables (zucchini is often a favorite), dried seaweed, and algae wafers.
Oh, and when I said daily water changes I didn't mean huge ones. Just something to keep nitrates low
. Even every other day would probably suffice but by doing them daily you'd see bigger changes.