JohnnyHero

You are referring to a common characteristic of entrepreneurial types. Depending on how self aware they are, you might be able to discuss things directly as they come up. You might frame things like this "I hear you telling me to give this new project my immediate attention, does that mean you wish me to deprioritize my current projects A, B & C?". A lot of times that context will jolt the owners brain and cause a reassessment.

On this flip side, this type of owner is often self obsorbed and unable to get perspective. These types will rarely be responsive to immediate feedback that counters their pet project. It is best to circle back at the end of the day and approach with a question of how they would solve the problem you have with prioritizing existing projects. Most people are able to "get real" about expectations after the inital heat of the idea has a chance to mature. I'd frame the discussion like this "Here are the challenges i've discovered while executing your new project, I need your input on how to solve or overcome some of these issues." When the owner is faced with the various hurdles that have to be overcome he/she may reassess the importance of the original project. On the other hand, don't create roadblocks just because you don't like the project, as it will be obvious and may cause them too decide that you are not the employee they thought you were.

relationships are difficult lol. however since you are an employee, it is your job to perform the duties given whether you like them or not. So either be a contributor or move on...