Octopuses have been found to host some parasites but the ones I have seen reported are in the gills and not visible on the outside. The ich parasite is not often hosted by scaleless fish (like mandarins) and we have not had a reported case of an octo hosting any observed parasites during my entire time on TONMO. The combined information makes me think an octopuse is unlikely to host them, however, ich is not a virus, it is a parasite, a living critter that feeds on its host and I don't know that there is ever a guarantee.
It depends on what type of ich is present in your system. There are three main salt water types - Cryptocaryon, Oodinium and Brooklynella. If you leave your system fish-free for the next 4-8 weeks, you should be safe. It appears that these parasites only have a fish host, not an intermediate host (like an invertebrate), so you should be ok. If it was my tank, I would wait 8 weeks - it may take you that long to locate an octopus anyway.