Alexander Morris -- who joined the band in 2019 as a vocalist -- filed suit against Michigan's Ascension Macomb Oakland Hospital ... explaining he was rushed by ambulance last April with chest pain and difficulty breathing, and when he got there he told a nurse and security guard he was a member of the famous Motown band.

In the docs, obtained by TMZ, AM says he wanted the hospital staff to know he was famous because of security concerns with stalkers and fans ... but he says a white security guard and a white ER doctor didn't believe he was famous and figured he was mentally ill.

Worse yet, according to him, they ordered a psychological exam instead of treating his clear symptoms of cardiac distress.

In the suit, Alexander claims he asked if he could prove his celeb status by showing his identification, but says a white male security guard ordered him to "sit his Black ass down."

Alexander says he was taken off oxygen and strapped into a restraining jacket ... and was only released from the restraints after 90 minutes, when he was finally able to show a nurse a video of him performing with the Four Tops at the Grammys.

Instead of getting emergency care, Alexander claims he received a lower standard of care based on his race.

Once the hospital realized who Alexander was, he claims he was ultimately diagnosed with a heart problem that may require a heart transplant, plus pneumonia .... and he says he suffered three seizures during his hospitalization.

And, get this ... Alexander claims the hospital offered him a $25 supermarket gift card "as an apology for the dehumanization and discrimination he faced at the hands of the hospital." He says he refused to accept the gift card.

Alexander is going after the hospital for damages. Interestingly, he's been making these claims publicly for a while -- doing interviews locally ... now, he's making it official in court.