"E + verb" is enough to be understood. "E hoʻomaʻemaʻe." == "Clean", as a command. "ʻOe" makes it clear who the audience is, and "i ka hale" shows what should be cleaned.
From what wiktionary tells me, ho‘o- is used to form causative verbs or verbs of acting. Ma‘ema‘e on its own means "to be clean, pure, chaste, attractive". When ho‘o- is prefixed, it means "to clean, to purify, to purge". So it has a more specific meaning of cleaning.