please does anyone know why coral have no medusa stage?

Answer 1

Probably because there is no need for it. Corals reproduce sexually, once the egg hatches, the larvae settle very quickly, or they reproduced asexually.

In both cases, the new polyps are pretty much in the optimum position to mature. Drifting as a medusa could take them away from coastal areas in to oceanic waters where they would almost certainly die.