The suspect, described as a black male between 50 and 60 years old, is seen inside the restaurant wearing a black fedora hat, glasses, black trench coat, and black pants.
The shooting happened around 5:25 p.m. in 3600 block of North Broad Street.
Police say the gunman was eating inside the restaurant while an employee was cleaning tables nearby.
The customer became angry, telling the employee to stop cleaning near him, police say.
"He was doing his job, but the guy kept saying, 'You can't be doing this spraying. I gotta eat. I gotta eat,'" said witness Keith Parker, who was eating dinner in the restaurant when the incident occurred.
Police say 19-year-old Josiah Myers, the brother of the employee who also works at the restaurant, attempted to resolve the conflict.
"He just tried to keep his brother from getting involved or having an incident. Basically trying to keep things from escalating," said Philadelphia Police Captain Nicholas Brown.
Police say the suspect left the store and then returned, shooting Myers once in the back. Myers then fell to the ground as the suspect shot him two more times.
"Bam. Bam. Bam. And the person he wanted to shoot, he didn't shoot. He shot his brother," said Parker.
Parker tells Action News the shooter placed the gun back into his pocket and calmly walked out of the restaurant traveling southbound on North Broad Street.
Myers was taken to Temple University Hospital and is now in stable condition.
Investigators have obtained surveillance video that shows the shooter had taken a SEPTA bus to the restaurant.
Meanwhile Parker, who says he's stunned by the senseless violence, is praying for the man he tried to save.
"My attention was on him. Stop the bleeding, compress. I learned all that from TV," said Parker. "I just hope I did something right. I just hope he'll be alright."
The restaurant was open again by lunchtime Monday.
Police warn that the suspect is considered armed and dangerous. Anyone with information is asked to call 911.