If the flowers are the same structure as a spider lily (Hymenocallis?), the likeliest scenario is that a spider lily bulblet or seed has somehow been mixed in with the Agapanthus, and this is its first year to bloom. If the flowers are the same structure as an Agapanthus, except that they are white, it is possible that a shoot of the clump has mutated--aka, a "bud sport"--into the albino, or white form. That is one of the most common mutations among flowers, since it simply involves disabling the biochemistry of color production.