Can you do something to help support the rock without moving it? It's true, octos are sometimes not the best builders. It's a good thing to have shells and lighter material in the tank so your octopus doesn't build entirely with heavy stones - and of course, make sure that your rocks are piled in a safe way. However, during den building your octo may move rocks, even large rocks, so you can never be sure how stable the structure will be.
Nancy