It's called the coconut ‘meat’ or ‘endosperm’. It is rich in phytosterols, cholesterol-like compounds found primarily in nuts and legumes.
But if you are pertaining to small seed like "ball" that floats around in the "coconut water" within the shell, it's called 'coconut sprout' or 'coconut apple'.
When a coconut germinates, a spongy mass forms inside the cavity, so there is no longer water and albumen, but a solid white ball with traces of buttery yellow exterior. When bitten into, it tastes deliciously of coconut.
Being rich in fibres and other nutrients; coconut also comes under the category of ‘Super foods’. Check out these 10 health benefits of the wonder fruit coconut shared by Theayurveda.org.