Probably the start of an ulcer. Unfortunately many ulcers in rainbowfish are caused by Tuberculosis marinum or other species of Fish TB.
You can try daily water changes and gravel cleans, and salt but it doesn't normally help.
Increasing the amount of plant matter in their diet does help to a degree, as does reducing the amount of frozen bloodworm in their diet.
Unfortunately, if it gets worse over the next month and turns into a big red ulcer, you need to euthanise the fish and don't add any new fish to the tank because it will probably be contaminated with fish TB.
If this si the case, you should avoid putting your hands or arms in the tank if you have cuts or scratches. Wash your hands and arms with warm soapy water after working in the tank. If you develop any small sores that don't heal, tell your doctor your fish might have TB and get the doctor to take a swab and send it off for culturing. Then find out what anti-biotics work on that particular strain of bacteria. Do not take anti-biotics until the swab has been cultured and tested in a lab.