Unfortunately, the very busied movement (along with a weakened immune system) is highly suggestive of senescence (old age). Treating with antibiotics will not harm, may help with the infection, but will not slow the aging process. I don't know it the excessive activity could be directly related to the infection only so attacking it would be my suggestion, regardless of the reason.
Call your petstore to see if they have
Fish Cycline. You can call CVS to see if you can get (and if they have any in stock) a human version without a perscription (explain it is for a fish - they will never believe octopus) but it is unlikely. If you can't find tetracycline, look at the medications that they do have (will not likely to be able to be done over the phone) and see if they have anything with Kanamycin Sulfate and/or Neomycin Sulfate (can be given together). These are the only drugs that have been (along with betadine for topical treatment in a QT) sited as having been used on octos (and are commonly used to treat other marine creatures like seahorses). If you find a cocktail antibiotic make sure there is NO copper but other ingredients are unsited as far a negative effect in our journals (to my memory and short search). If you are going to treat it is important to try it immediately as if the infection worsens, he won't eat and will be impossible to medicate.
Here is one discussion on trying to give tetracycline.
I am thinking hummelincki (do you see the purple/blue rings around the suckers I mentioned?) because of the arm length but finding the eyespots (or determining there are not any) would help confirm or deny my guess. I still think the arms are too short for aculeatus (but they can throw them to escape preditors so length is not always a go by) but we occasionally animals without eyespots that are similar to hummelincki but come from Indonesia or the Phillipines and have never labeled these with a species. Look at the List of our Octopuses (Forums->Journals and Photos then top of page) for 2008 and 2009 and scan for hummelinlcki then look at the posts and photos to see if it helps with an identification.