If one person, all on his own, has a a clear belief that Reb Moshe Feinstein is vadai Moshiach, is he violating anything or doing anything wrong?
If he is unaware of the Rambam (etc.), he’s guilty of am haaratzus. I don’t think it’s kefirsh per se.
If he is aware of it, but insists that he is moshiach because he declared himself so, he is guilty of distorting the halachah and halachic process, as well as of being m’vazeh R’ Moshe by saying something unbecoming.
Probably, he’s just insane, and pattur as a shoteh. I think it’s ridiculous to think tens (hundreds?) of thousands of Lubavicher chassidim are all shotim in the literal sense; it’s just a hashkofoh gone horribly awry.
And, when such a large group distorts the halachah and the halachic process, it is a whole lot more dangerous than when a yochid does it, although they’re both avlos.