Is a hidden camera in my hall legal?

Talk to the landlord. It may violate the lease. Also, if it is in the floor, then it may record places where you expect privacy (up skirts for example). That could be criminal. See if you can get the landlord to deal with it.

If the hidden camera was installed with your landlord's permission, there is probably nothing you can do about it. However, it is quite possible that your neighbors violated their lease if they installed the camera without the landlord's position. Further, if the camera was installed without the landlord's permission and it photographs areas your apartment wherein you have a reasonable expectation of privacy, your neighbors may have engaged in both civil and criminal wrongdoing by installing the camera. If you want to pursue this, then you should first retain counsel, ask such counsel to record photographs of the camera, and then ask such counsel to discuss the situation with your landlord. Then, you should ask such counsel to attempt to negotiate a settlement with the neighbor involving removal of the camera. If such a settlement is not appropriate, your counsel should advise you as to whether to initiate a civil action or file a criminal complaint.