@Nekomamushi @GrandmasterChef Zonji
When we first see it, Robin points out that 200 years ago was when the fishmen first went to the Reverie. My guess at the time was that it was previously on Fishman Island to guard Posiedon, and Posiedon leaving for Mariejois activated it. It last ran out of energy before it completed its mission, which would explain why it's saying sorry - because it thinks it failed to protect Posiedon. Still holds water imo (no pun intended). Though the reason it is apologising may be deeper.