This argument that touching things is causing creatures harm because it removes the animals slime coat is debatable, IMHO. If that were the case, wouldn't they refrain from touching you? I've been buzzed by both mantas and eagle rays. Stuff down there regularly touches each other and the surfaces below. Doesn't that also remove their slime layer?
It might, and then again it might not, it depends on what type of surface and how hard the contact was. Yes fish come into contact with each other and also things in their environment. Our skin tends to attract mucous type stuff, and we do remove the slime coat. There are alot of articles in aquarium magazines, on teh subject of slime coats, what they do and what affects them.
Regardless, what fish do or do not do on their own has no bearing on whether or not we should handle them unnecesarily. Don't forget fish and crustaceans bump into or crawl all over coral too, yet when humans do the same we destroy it.
And then there is the psycological stress we put on them when we reach out to grab or touch them.
An easy way to remember what they might be thinking, wildlife lives by the bigger than me rule. "That thing is bigger than me, it wants to eat me".
Lots of people get a kick out of playing with damsel fish too. Well, did you know you were seriously stressing that fish out? Divers tend to hang there longer than other fish that it tries to chase, and sometimes signal to thier buddies to come over and play too. Now there are two large predators it needs to chase away, which both frightens it, wears it out physically, and distracts it so that some other predator may move in and swipe some eggs, etc. And it can be so quick and so subtle you have no idea what you just did.
People like to harrass blowfish, pull seahorses away from their perches, try to flush something out of it's hole, etc. It all adds stress they do not need.
Everybody touches things from time to time. Sometimes you can interact safely, sometimes what you do leaves the fish vulnerable down the road. I try my best but sometimes I just get so excited by what I am seeing that I can't help myself. But that doesn't make it right.
Personally I think we have done so many harmfull things to Nature that we should really be trying harder. And not touching or harassing what we see is a great place to start.
Lots of people also think it is ok to stand on the reef, after all it is only rock, right?