Keppel Island is currently experiencing complete coral bleaching, which is mainly attributed to the expansion of starfish. Coral bleaching is a phenomenon that occurs when coral reef ecosystems experience stress, such as increased temperatures or pollution, causing them to expel the colorful algae that live within their tissues. This process leads to the corals turning white or "bleached."

Starfish, specifically the crown-of-thorns starfish (Acanthaster planci), have been identified as a major factor contributing to the bleaching of Keppel Island`s coral reefs. These starfish feed on the living coral polyps, causing significant damage to the reefs. The expansion of starfish populations can occur due to various factors, such as disturbances in natural predator-prey relationships or excessive nutrients in the water, which promote their growth.

It is important to note that the other options provided - blue whale, octopus, and sea horse - do not have any known direct connection to coral bleaching on Keppel Island. They are not involved in the process or dynamics that lead to coral bleaching in this particular region. Therefore, the correct answer is option 1, starfish.