I have never witnessed, or heard of, any cephalopod getting parasitic infection from other fish in the tank. I may be wrong, but I believe that both Icthyopithrius (sp?) and crytokarion (sp?) are only found in vertebrates. Icthyobodo has been described in octopus (Forsythe et al., 1988) From the literature, the most common problem of captive octopus seems to be bacterial infections. I would not be very alarmed if I observed any fish to be infected in a cephalopod tank. I have a couple good references that I will get to you when I have some more time.
I hope that answered your question.