I don't recognise that plant but with the way it flowers it might be a Euphorbia of some type. It's surprising that a "weed n feed" would actually kill off Kikuyu, unless you used it at too high a concentration. They usually affect broad leaf plants. But obviously this plant had seed in the soil and without competition from other plants it's taken the opportunity to take over. The weed n feed has probably diluted by now and is feeding it. Depending on the size of the area, hand pulling might be the only effective way of eliminating it.