Is speaking to a dead person forbidden or polytheistic?

The prescribed action when visiting graves is to greet their occupants and supplicate for them, not to converse with them. The Prophet, peace and blessings be upon him, taught us what to say when visiting graves, which is to offer salutations and make supplications.

Whether the dead hear or not, conversing with them as one would with the living is not sanctioned in Islamic law. It can lead to detrimental consequences, such as becoming attached to them or believing they possess knowledge of the unseen, and it may even lead to supplicating to them or asking things from them.

Had conversing with the dead been a good act, the Prophet, peace and blessings be upon him, and his companions would have done so, as the religion has been perfected. Therefore, anything that was not considered part of the religion then, is not part of the religion now. Thus, the visitor should limit themselves to what is prescribed in Islamic law: they offer greetings, make supplications, and then depart.

If a person does converse with the deceased, it is not an act of shirk (polytheism) or haram (forbidden), but it is not prescribed and could potentially lead to something prohibited, so it is better to avoid it.