Why is the chocolate chip starfish always at the top of the tank?

Most starfish graze on film algae, and tend to go to where there are high quantities of algae.

Chances are your tank has lights, and the algae will grow stronger closer to the light.

The other reason is the starfish could be getting bullied by other tank mates, so it's just getting a break from the others.

Look at your tank at night with a torch, the starfish should be down the bottom grazing while the other fish sleep.