Can a police officer seize contraband lying in plain view on someone's desk? If so please explain?

It is not a question of "can he" he has to, it is his duty to do so, failure of him to act is punishable by his department and perhaps the court system (depending on the circumstance).
If it is in plain sight and it is illegal to possess in that state or county then the officer has probable cause to seize and doesn't need to ask permission to search.