If the person who spins Merillia is upset, then their Merillia becomes weaker and weaker. Without a change, the Merillia slowly poisons Oceana. When Queen Calissa was imprisoned by Eris, her Merillia was not strong enough for the sea to remain healthy. It only improved when Merliah released her.
Merliah gets distracted when at the end of the movie, she has gotten the power to make Merillia