扫黄 is a verb-object compound (离合词) meaning 'to crack down on pornography' — literally 'sweep yellow' (yellow = obscene). As a verb it can be split: a measure word or duration goes between the two characters (e.g., 扫了一次黄 'did a pornography crackdown once'). It is also used as a noun referring to the anti-pornography campaign itself (打一场扫黄 'wage an anti-porn campaign'). The two parts are inseparable for the noun sense.