if poliece come to my home on unrelated matter and take marijuana. but say nothing can i be charged?

If they legally obtained evidence, they can use it against you without you saying a word. They don't have to arrest or charge you at the time. Obtaining the marijuana in plain view on the spot likely would be grounds for a warrant. They can come back and do a proper search. Then they'll arrest you.

You're ******! They took your weed to use as evidence to arrest you

I think if they took it they took it as evidence. They don't just take it away the way you do a kid. It is possible that you will not be charged, depends on the unrelated matter, but I agree that he would have said what you are being charged with. I don't know if it is a requirement of law--but it ought to be.

Certainly.

Yes, they don't have to charge or arrest you on the spot.