A spa on Billionaires’ Row has been accused of operating a seedy brothel, advertising “Sweet, sexy, hot Asian” women, according to a complaint filed by the city. 57 Star Beauty Shop, which abuts supertall 111 W. 57th St. — currently advertising a $98 million listing — has been the target of two undercover police stings since December. “It was somewhat of a little known secret,” said a former fourth-floor tenant, who relocated about three years ago. “It was obvious. It was disguised as a massage parlor, but it was never open.”