You are called a prophet when you are someone who is in contact with a god and speaks on their behalf. I'm sure you know the story of Joseph Smith at age 14 praying in the grove. I guess when you make up your own new religion you can be whatever you'd like, or call yourself whatever you'd like. Prophet? Apostle? President? How about all of it?

Not to crap all over anyone's religion, since LDS Christianity is not all bad. However, to evaluate a prophet, one must inspect that prophets predictions and claims and measure those against observed science and history. Joseph smith prophesized that matter could not be created nor destroyed, which is in direct contradiction to mass-energy equivalence (E = mc2, or, more properly E2 = m2c4 + p2c2). Smith prophesized that exactly one person at a time would hold his prophetic power, yet, when he died, he left no clear successor, and no one prophet has clearly continued his prophecy clearly. Perhaps the most demonstrably questionable thing was that Smith reversed his position on slavery during his lifetime. On its own, that's not particularly bad, but Smith used his status as prophet to justify both positions. At first, claiming that God had revealed to him that black people were descendants of Cain, and thus cursed into slavery by God. When it was pointed out to him that the descendants of Cain would have all been killed in the flood, he revised his position that black people were descendants of Ham, the son of Noah, whom the latter had cursed, and doubled down on his position that slavery was not only justified, but prescribed by the Bible, and therefore anyone who opposed slavery would be subject to God's wrath. By the 1850's, Smith disavowed his own position, saying that the slaves should be freed and the US government should pay their former masters for their freedom. The modern LDS church is very distant from that original position, encouraging membership across all races, although some split-off groups, such as the FLDS church still teach racism. At any rate, any prophet who reverses a prophetically-inspired teaching as drastically as that fails the test. It's very basic logic. Assume God exists. Assume prophets exist, who speak the Word of God when they indicate such. Assume false prophets exist, who indicate that they speak the Word of God, but do not. If a person claims to be a prophet, and claims, under prophetic authority, that God's position is A, then later, claims that God's position is ~A, either God's Word was false or the prophet was false. If God is infallible, then the only uneliminated option is that the prophet is false, thus, Joseph Smith was a false prophet.

Who said he was? Who is he anyway? I don't know him.

He isn't.😏