Concealment is hiding something or someone. What is concealed may or may not be protected. Perhaps it is merely hidden. Concealment is something done to prevent disclosure, usually a single action.
Sergeant Jones wants to conceal his truck from aircraft, so he hides it in a garage. But if he had wanted to protect it too, he would have put in a bunker. Concealment means hiding. We rarely speak of a level of concealment.
Now he has a plaintext message. He is going to conceal it inside a photo and send it to the friendly local militia. If he had wanted to protect that message, he would have encrypted it.
Secrecy is a practice, the state of holding secrets, between people. Everything that Sergeant Jones does is done in a climate of secrecy vis-a-vis a specific threat or threats. We can then speak of levels of secrecy. Secrecy expresses overall security needs--the big picture of relationships, trust, privacy, and disclosure.