Muriatic acid should do the trick but you need to protect your skin and use old clothing while doing the cleaning.
I hope it is outside! Because both suggestions have are messy and recommend for outdoors ONLY! OPTION1 use a strong pressure washer. With just water. If there is some paint left add dawn right to the paint with a hard bristle brush scrub. Then repeat with washer. OPTION 2 sandblast it. I promise this option works a100% of the time but it's NOT for thee beginner DIY. I hope your paint was applied to the surface. Otherwise clean and paint over it.
It might be possible to chip it off with a putty knife unless the concrete is textured. Otherwise see other people's answers. You might be able to use a wire wheel in a drill to carefully grind it away. Wear safety glasses.
I would try Goof Off first