While not officially handing in his resignation, Fr Alvaro Corcuera has asked for and been granted a "sabbatical", handing over his duties as General Director to Fr Sylvester Heereman until the next General Chapter.
My take. He is resigning. The next General Chapter will elect the new governing body of the Legion and Regnum Christi, and may just elect a new General Director. Therefore, in effect, it is a resignation.
You can read Fr Alvaro's and Cardinal Velasio de Paolis' letters here.