My supervisor is trying to "steal" my collaborators: what can I do? I work as a postdoc in Prof. X's research group. I have my own funding and work on my own topics, which are related to X's interests. X is an expert in experimental work and I do simulations, on which X lacks expertise.

X largely depends on me for the simulation projects to progress, including doing the supervision of students working on computational topics, who X cannot supervise directly since he lacks the required expertise.

I have a very productive line of collaboration with a group of researchers who are world leaders in this particular topic. X has been somewhat involved in our joint papers through me, benefiting from high impact publications, although he has not contributed significantly. My collaborators are mostly interested in working with me because of the expertise I can contribute to their own.

Understandably, X is trying to minimize his dependence on me. At the same time, he wants to keep collaborating with the top team on the projects that I have been leading. So he's contacting them to write proposals together while keeping me in the shadows. I have no problem with this, everyone is free to work with anyone else, except that my collaborators seem to be assuming that this is just a continuation of the work with me, and that I am involved. Awkward situations have arisen where one of my collaborators has cc'ed me in a reply to X's request to write a joint proposal, assuming that I was involved in it, while I was totally unaware of what was going on (since X has not been in contact with me about this). Another collaborator with whom I talked to in person recently was also very surprised that X had not let me know about this proposal, especially because he's promising to continue the research that I am currently leading and which is based on my ideas, codes, etc.

Incidentally, I am preparing my own project proposal (of which X is fully aware) for the same call. I will be listing the top team as collaborators and some of the topics will overlap with X's proposal. Since we will be competing in the same review panel, I am afraid that the funding agency or the reviewers will discard my application on the basis that the work is going to be done on similar topics and with the same collaborators as X's project and, since X is a professor and has a stronger overall profile, X will be better at taking the project to completion.

I feel that, in his understandable effort to loosen his dependence on me, X is trying to steal my ideas and collaborators, and in the process reducing my chances to advance my career. Is there anything I should or could do to solve this problem, such as letting my collaborators know that X is purposely leaving me out of these proposals? Or should I just concentrate on writing the best proposal I can and hope that the panel with agree that I am the right person to do it (this is my current strategy)?
Leave this place immediately and find better support elsewhere.

As a postdoc you're independent and well-aware of it. You have your own network of collaborators, reputation. You know your job better than others. Nonetheless, some bureaucrat who depends on you is openly sapping your career, and you find that "understandable" and believe there's a friendly way around so that everyone is happy. I think you hold all cards there. 

My advice to focus completely on networking and finding a new place. The situation will not improve. Make use of the time and resources you have now to take the best advantage of your freedom to move and communicate. Why not move closer to your collaborators? It's fairly easy finding another postdoctoral position nowadays, visas, etc. There's no point in holding such bad ground. 

Networking will also naturally broaden your horizons, through revealing other career possibilities which steer away from academic parasites. Whatever you decide, just don't allow some parasite to win a game where you hold the cards.