(Okay, before I get into the Uma stuff, I do think it would be really cool to explore the dynamic of the actual laborers who are getting the oysters and mussels being from the Isle. Like, the Auradon guards are instructing and inspecting them, but they're not doing the dirty work. I'm sure that was obvious from what you already said, and I'm not really expounding on it or anything; it just seems interesting, the idea of this becoming a privatized thing. Before, they were collecting oysters and pearls for themselves, but now they're earning a wage collecting the oysters and pearls for Auradon. And of course the wage they earn is way less than what they would get if they could still keep the oysters and pearls for themselves. Okay, I'll move on now, lol.)
The guards' job is definitely just to guard the local bivalves and prevent/handle crimes pertaining to that trade, but these are Auradon guards visiting the Isle of the Lost; they're looking for any opportunity to be jerks in ways they can't at home.
The most persistent problem from them (other than rudeness, which everyone on the Isle is used to) is that they like to "confiscate" or "search" the possessions of anyone who walks past them, on the docks, and then not return them. After all, it's not like a VK has anyone to really complain to if an Auradon guard wants to keep their wristwatch.
Unless of course said VK happens to belong to a pirate crew.
Uma confronts the guards publicly, at first. Demands to see everyone's possessions returned.
In response, they smugly demand to search her clam shell necklace. Harry nearly attacks them right then and there, but Uma calmly hands it over, lets them make a show of searching it.
(She does sarcastically remark, "What, you think it might be made of oyster?")
And when they say that they'd better hold onto it, just to be sure, she lets them, holding Harry back from lunging at them with a breezy comment that there's too many of them to fight.
Really, her purpose was to see where they'd take it. She has some of her stealthiest pirates following them, to figure out where everyone's stuff is.
Late that night, the captain of all the visiting Auradon guards retires to his cabin one night, only to find there's someone waiting for him there.
"Captain," the shadowed figure greets him. She sounds like a child. He turns on the light. Looks like a child. But she sits in his desk chair like it's a throne, and there's a sword laying across her knees, her hand on the hilt but not gripping it. "I regret the late hour, but I hear some of your guys got some sticky fingers. I'd like to discuss it."
The captain smirks. "I've heard of you. Ursula's daughter. So, you're here to threaten me?"
"Captain Uma is my name, sailor, and if I was here to threaten you...well, let's just say you'd know it. But I see you got a chessboard here. Let's say, you play me, and if I win, you return what your guys stole."
Uma smiles a dangerous smile, not liking the way his eyebrows arched. "If you win, then you don't return what your guys stole."
(Of course, she's just distracting him. Regardless of whether he wins, her crew is already liberating the stolen goods. Which isn't a breach of their agreement, because she only said they're playing for whether he has to return them.)