As a general rule it really isn't the best thing to touch your fish as it can rub off their protective slime coat. An occasional touch or bump won't really do any damage, though. The only time I do it on purpose is if the fish is sick, lethargic and I don't want to net him, as he would be more used to my hand in the tank than a giant green net.
As for the "stretch mark" do you have a photo?

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