I keep thinking we need a sticky about successful tank decor. You should be keeping low lights so that is a consideration in addition to stinging and uprooting.
With the lower lighting (I only use LED's - 100 of them in 5 dome lights attached to a board) serpent stars (OK so it is not a coral) are more enjoyable than in other tanks because you will see more of them. They come in a variety of colors from red, to striped to yellow (common lore is to avoid green and I know there are some agressive greens but I have one that is not but is of a different variety than the ones that get large).
Feather dusters will not harm the octo. The jury is out on whether or not the reverse is true. My pencil urchins ate my feathers - as well as a non-photosynthetic gorgonian - when there was not enough algae (imagine that) in the tank to sustain them (I now add sea panseys every couple of months but did not add new feathers). Nancy has read reports that some octos will decide to decorate with the feathers causing their demise.
Red mushrooms work well in the low light (they won't get huge like the one in my reef tank but they will live a very long time) but will look more brown than red. My dwarf never bothered the couple I have in there.
There is a nonphotosynthetic sponge that is a light bright orange (Ken calls them frilly sponges but I don't have their proper name at my finger tips. Check with Dan or try www.sealifeinc.net to look them up.) These are the only sponges I have ever been able to keep for a long time and they seem indestructable unless they start getting covered with algae.
For additional color, you can try the purple and white barnical shells. These do well in the aquarium and may serve as a den for the octo. Be sure that anything dead you buy has not been treated or coated to "preserve" it. Clorox treatments are ok as long as it has been declored for at least 3 weeks (you may want to do this with anything purchased dead to be safe. Anything picked up on the beach you should consider a 2 week clorox bath followed by 3 weeks of declorination - old school SW tanks ).
Sea Apples will wipe all life in your tank out if they get stressed. They dispell their guts releasing a toxin. I know all about it. It wiped out our Seahorse tank. Must have gotten damaged on a powerhead. An octopus would surely stress it out.
Carnation corals do prefer low light areas, but they have a VERY poor survival rate in captivity, they require a high nutrient diet, constantly, making your water quality terrible for an octopus.
Trumpet and Candy Cane are the same coral. They both have stinging tentacles. Tube and Candy Cap I'm not sure about.
I think we're approaching this from the wrong direction - since the beginning we've said that an octopus should be kept in a "species" tank - your octopus alone.
Then, when we think what we could add that's safe and reasonable, we come up with:
a small pencil urchin
small hermits and snails (your octo will get to the point that he's eating larger food and will leave these alone)
small shore shrimp
mushrooms (put them in protected places)
small things on your live rock
a brittle star or serpent star (not green) of reasonable size
Notice that fish are not on this list. Fish can bother an octopuses eyes and make him stay in his den, Or, they soon become dinner.
It's possible to have a beautiful tank by arranging your life rock attractively and alllowing for many den sites of various sizes, making sure you have good coralline algae growth, adding some colorful shells and even a few glass marbles. Barnacles are attractive, too. You can use flowerpots as dens or pvc pipe, if you choose. Some have even bought resin decorations, such as sunken ships for their octopus tanks.
If you want more lively activity in your tank, buy a few one-inch shore shrimp. They move quickly and it's unlikely your octopus will be able to catch them for a long while.
Do pay attention to the size of the tankmates - don't have a tiny octopus and a large serpent star.
Well, if you feel you really have to experiment with adding other things to your tank, I would say I don't think they're dangerous. They stay retracted into the rock most of the time. On the other hand, they require direct feeding with a baster or pipette, and they usually only open their polyps at night, although some will begin to respond to feedings during the day. This however, could be detrimental to your water quality, and in turn, your octopus.
It really is best to keep it as simple as possible. There is a large variety of mushrooms, in just about any color imaginable, and they are easily split/fragged to make more.