Moon Spell Control is a skill that merpeople know, it lets them survive the full moon without letting it control them. According to Sirena, merpeople learn it when they are hatchlings.
In "Outcasts", Nixie, Lyla and Sirena are in Moon Pool to contain the light of the full moon. But Zac ends up falling in the pool, disrupting the process.
In "Reunion", Evie is concerned that she will fall under the moon spell again. But Sirena, Mimmi and Ondina decided to help Evie with the moon spell control. Sirena helped Evie through the process.
In H2O: Just Add Water we can see that Cleo, Rikki and Emma do not know the existence of this process, so they end up being bewitched by full moon constantly. Bella has not been shown to be bewitched by the moon because with the arrival of the comet, the Moon Pool's properties have completely changed, but she might of been fallen under the moon's spell earlier in life.