You may target a creature you already control with Roil Elemental's ability. This will usually have no visible effect, but it will overwrite any previous control-change effects. For example, if you gain control of Roil Elemental until the end of the turn (with Mark of Mutiny, perhaps), then its landfall ability triggers and you target Roil Elemental itself, you'll gain control of it indefinitely (since its control-change effect will last for as long as you control it).