Bermuda grass, and the only way to kill it is use a herbicide . It spreads by Rhizomes. Rhizomes are modified stems running underground horizontally. They strike new roots out of their nodes, down into the soil. They also shoot new stems up to the surface out of their nodes. That makes it impossible to pull them by hand because all of the nodes break when pulled.
Contact kill herbicide Glyphosate will kill it, but you have to treat the area 3 times. One spray won't work, because the rhizomes that were missed regrow again. Spray one time, then regrow the grass, and spray a 2nd time, then regrow the grass again, then spray a third time. That will take a year to kill the Bermuda, so the following year you can plant your good grass and not have any Bermuda.