It is an air helmet with a pink and yellow flower on the upper-left side.
Role in series
Sandy is required to wear this air helmet when outside of her treedome to make it so that she won't die from a lack of oxygen, as seen in "Pressure" where her lungs deflate after being challenged by the sea creatures to breathe without her helmet and she quickly puts a pickle jar over her head to recover.
However, when in her treedome, she removes the helmet, as well as her astronaut suit and boots, since her house is an air-polluted area of the ocean that doesn't require such equipment.
The first episode where Sandy was seen without wearing her air helmet in the ocean was "Pressure."