dead and deceased - no difference - deceased sounds more polite and is used in formal and legal language

It's deceaseD, not decease. "Dead" is more natural and common. "Deceased" is a polite term or euphemism (委婉语) and is only used in very formal situations, for example, in newspaper obituaries or at funerals. Also, "dead" and "deceased" are adjectives. "He's dead." "She's deceased." The verb form of "dead" is "die." (He died last night after a long illness.) There is NOT a verb form for "decease." Instead, to be polite you can say someone "passed away." This is the same as 去世了.

No difference ,both means someone who is no longer alive.

They both mean the same thing but deceased is used almost exclusively for humans. The word dead, however, can refer to humans, animals, or artificially animated objects such as batteries or telephone lines.