I would introduce myself and talk to the other owner. You can be polite and friendly but still communicate your desire to keep a tight ship and not have the boat visited while you are away. They may try to tidy up. I am a messy guy, but I do try to keep things neat in shared areas out of respect for others. I'm not stupid, but sometimes I need someone to point out that something is shared space. He may not realize that the standard is higher for slips than for a personal dock on private property.
Newport 17 - "Kohanna"
At sea Darwin's hypotheses is the final arbiter of right of way.