I have those too. Everything in the tank avoids it like its fire! I have seen Legs touch it but she just flinches and moves the arm touching it. I have never personally touched it but a friend told me they will sting humans pretty bad.
That looks like aiptasia. It does bother octopuses and is quite a nuisance. I was able to eliminate mine by injecting boiling water into the center with a syringe (Colin's method). However, I had to have someone else distract my octopus so that she wouldn't get stuck by the needle.
There are preparations that can be used, but I don't know how octo safe they are. Joe'e Juice is a good one, and recently I've used Aiptasia-X by Red Sea very successfully.
I don't think I've heard of aiptasia stinging humans, but it might. Most people eliiminate them before they grow large.
The LAST thing you want to do is cut/tear/ rip them !! The tiniest piece will grow into another. They are a real pain. Either get a syringe and inject them with boiling water like Nancy said, or you can also use lemon juice.
Don't know if it would be feasible for you, but peppermint shrimp will devour them. I had aiptasia everywhere in my reef tank, bought a half dozen of the shrimp and within a couple of weeks there was not one to be found. You would need to put the rock/shrimp either in your fuge or a separate tank, cuz I don't the shrimp would last very long with your octo.
Aptasia are a nuisance that you should try to control and they spread like crazy. I have had the displeasure of having two kinds. One is relatively easy to control (Joe's Juice is my choice for initial eradication) and does not seem to effect humans. Once I put two of these in a 1.5 gallon just to see what would happen, there were hundreds after several months. Then there are the ones that have a larger central disk and do have a sting you can feel and are much harder to eradicate. I have these in one of my mantis tanks and every time I think I have killed them, they return. The only ones I have been able to eliminate are those I have been able to suck up in a turkey baster from the bottom substrate or after hitting them with one or another of the aptasia control formulas.
Peppermint shrimp will eat the newly formed and keep them controlled if you can keep one alive but don't seem to eat the ones with a central disk much larger than a pen point. There were two peps in Beldar's tank when she arrived. One is still on aptasia patrol, the other was too curious about an octopus.
Well i tried the boiling water and syringe and it kinda worked. It seemed if you didnt hit it right in the middle of it it wouldnt work. I am going to try again today. I am thinking of getting the red aptasia x. Does any one recommend it? I watched the video of it and it did actually make them implode.
Just to throw this out there... if it is stinging you, it may very well be a curly Q anemone. The arms look as if they have a "spiral" apearance in the second picture. I deal primarily with corals in my tanks, and I have never once been affected by touching an aptasia, and I have huge ones in my refugium (they are great filter feeders). Now thats not to say that you may just be mildly alergic to them and so felt the sting.
If you happen to have kalkwasser powder (calcium) handy it's the same thing as Joe's Juice... you can buy enough to dose your tank for calcium many many times (if you have a reef setup it's more practical) and it's cheaper than a tiny vile of Joe's Juice. You can make a paste out of it and a mixture of water from your tank, suck it up in a syringe, feed it to the anemone/aiptasia, and "So long sucker". They melt.
If you use a ton of it, it can throw off your pH, but just injecting a few won't hurt anything and it works every time.
I tried the home brew method with the ones in this tank (kalkwasser, calcium hydroxide, is pickeling lime and you can buy a pure can of it really inexpensively at the grocery - MrsWages states on their website: Food grade calcium hydroxide with no additives or preservatives.) I am not sure if JJ is ONLY kalkwasser though. Either these particular anemones are resistent to PH change (the result of using kalkwasser) or JJ has more in it. One thing to note though, if using JJ or you are making your own brew, I have found that microwaving the mixture helps keep it in suspension. I will report back after using JJ on these critters (I had not tried my own concoction before) to see if I see a difference.