They are opportunistic feeders. They hunt because they are hungry. I think the biggest mistake people make is to put serpents into a new, non-established system. If there is nothing for them to scavenge, they will find food, ie, your fish.
I have a couple in my tanks, banded serpents, not greens, that I personally enjoy alot. They know when the food hits the water, out of the rock come the arms and I feed them bits of shrimp. They are little beggars. I have one that is pretty large and wonder if he can come out of the hole he is in as I have not seen him out in over a year.
I also have bristles, they are more "business" and do a great job.
I can't say that my serpents make a huge impact as part of the cleanup crew. I have them more because they are particularly striking specimens and I choose to feed them (which does take away from them scavenging) in order to enjoy them in my systems.