I would consider removing any loose materials from site and worrying about the legalities later. What are the terms of your contract with the FM company regarding ownership of work done but not paid for? Do you retain ownership until paid, sometimes known as a Romalpa clause?
If you did remove everything from site and the FM remains solvent, you may be kicked off the job. Would that be a larger long term loss than the current sums at risk?
No doubt somebody with legal knowledge will be along shortly, or somebody who has had a similar experience previously.