Yes. They created a public nuisance. Otherwise, this thread would not exist.
And I would not feel "busted." THEY are the ones that made an unseamanlike error. I would explain very politely that the slapping halyard was chaffing and that as a good neighbor, I felt an obligation to prevent damage. This is also true. Many sailors don't know their halyards slap, since they aren't around when it is windy. I learned to secure mine properly after someone secured mine. I felt embarrassed, not violated, and I never made that mistake again. So I learned from it.
In my experience, telling the dock manger is a total waste of time. There is zero chance they will respond to a slapping halyard; most of the boats "zing" in a breeze and they don't need confrontations. They might respond if a boat was loose and striking other boats. Might. If a boat is clearly sinking, the most they normally will do is make a phone call, and then wait until it is on the bottom. Liability, man. And the opportunity to bill for work.
A more interesting question is whether you would adjust another boat's lines.
a. Not near your boat. Only likely to damage itself. Might mention it to the marina.
b. May damage some other blokes boat. Would mention it to the marina.
c. May damage my boat. Would do it right myself, then call the guy myself, if needed.
Morally, all three questions should have the same answer, but they don't.