From my experience, he only appears once a day and always on the first scallop. If your inventory is full and you have to toss something, he won't appear.
If it seems like he didn't give anything, check to see how many pearls you have, or he also has the chance to give mermaid clothing.
If it seems like he didn't give anything, check to see how many pearls you have, or he also has the chance to give mermaid clothing.
