THe fish you are talking about should not be kept together under any circumstance.
This is what I sent him :
Plecostomus are tropical omnivorous fish that need a wide variety of vegetable in there diet.
Koi are cold water carp. These two fish should never be mixed together for that sole reason. When a pleco is hungry(algae only makes up 20% of there diet) they will suck slime coat off slower moving fish, carp are slow moving.
Just separate them and research dietary needs of your fish and you will find out how to get your pleco to behave, a full pleco, is a happy pleco.