Some TMNT stuff really isn't for little kids.
In the 2014 live-action film and its 2016 sequel (which is a reboot of the Teenage Mutant Ninja Turtles film series), the Foot Clan is a criminal organization and a threat to the citizens of New York.
Unlike their previous incarnations, who were described as a international, all-ages, ninjutsu cult, the Paramount version of the Foot Clan is now classified as an adult, modern American domestic terrorist organization and besides the Shredder. None of its members are versed in the traditional martial arts. Besides Shredder and Karai, the film also introduced Eric Sacks who is Shredder's student, Dr. O'Neil's former lab partner, and the CEO of Sacks Industries. Here the Foot Clan was so named because its members "step over the good people of this city with no regard". Instead of black unitard-clad masked ninjutsu warriors, the Foot Soldiers are portrayed as fully armed men that wear black military-like uniforms and often wear kabuki style masks on their face in order to conceal one's identity.
In Out of the Shadows, more traditional Foot Ninjas make their appearance, though they are updated with modern body armor and technological advances that allow them to do things like crawl up vertical surfaces.