The scroll shaped button on the right hand side of your screen is the menu for your missions in Splash.
Missions are special tasks given to you by your Turtle and Hermit Crab, or it could be a Daily Mission. When completed you will be given special rewards.
Missions can vary, including things like being tasked with having a certain amount of species in your reef at once, creating a school of jellyfish, collecting coins with a particular fish etc.
Once your mission is completed a green tick will appear on the scroll on the side bar indicating the mission is complete.
Tap on your missions icon and then tap collect to receive your reward. Once your reward is collected you will be given a brand new mission to complete.
There can be times when you have now missions from the Crab and Turtle at some points but you will get more missions when you level up and unlock more species.
Crab and Turtle missions can not be skipped so if you are having trouble with a particular mission, please get in touch with the support team!