Hi Gabriella, It's exciting you are looking for a PhD program! I don't think they are doing much octopus cognition at MBL. Your best bet down there will probably be Roger Hanlon but I am unsure if he is taking on any PhD students at the moment. You could also reach out to Josh Rosenthal but I know his research is primarily focused on cuttlefish epigenetics. Bret is mostly focused on animal husbandry and doesn't lead much research on his own but he may be a good resource to connect you with other researchers in the area. Carrie Albertine could be a good person to contact as well.
You could also try to get in touch with Eric Edsinger or Gul Dolen- they have both done octopus cognitive work in the past, but I am unsure what their current status is.
Let me know if you have troubles finding contact information for any of these people and I can help.