I assume you've done the research to know what a monumental undertaking this will be so I certainly won't try to dissuade you.
Don't count on weight to anchor your rocks. Drill them and connect them with acrylic rods to secure them together. Octopi (languages change. Regardless of the origin, this is the most accepted plural today. But good for knowing the etymology!!!) are incredibly strong and agile. They can topple rocks you would expect to be far too heavy.
They are also incredibly intelligent and an argument could be made for sentience. Not only will your tank need a nearly airtight lid, the overflow will also need to be escape proof. I'd suggest a screen rather than traditional slits for the overflow. It will need to be cleaned daily, I imagine, to avoid clogging.
In addition, once the octopus is acclimated and used to your presence, it will need a lot of gentle interaction to avoid boredom and depression. Toys and mirrors may also help.
As for macro algae, go for easy and fast growing types. Caleurpa, chaetomorpha, etc. as these will also provide supplemental filtration.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef