It should correspond to U. horizontalis (though it is not very typical and not 100% sure). It seams that the id feature that if the flowers occupy more than half the flowering stem is not really valid anymore, but the dimensions of the corolla/bracts are more important. In fact at first I placed it as U. rupestris, but then the dimensions/shape of the corolla did not fit well for U. rupestris
Are there any other species to consider. Apart the branching, the corolla is a bit un-pigmented in yellowish-green. There is a certain Umbilicus chloranthus which would be a new species for Malta, but for now I better dream of a White Xmas rather that!