Etymologia quidem certa non videtur:
Probably from Spanish or Portuguese alcatraz (“pelican, gannet, albatross”), probably derived from Arabic الْغَطَّاس (al-ḡaṭṭās, “the diver”); or from Portuguese alcatruz (“water wheel bucket”), from Arabic الْقَادُوس (al-qādūs), from Ancient Greek κάδος (kádos, “pail, jar”), in reference to the pouch of a pelican.[1] In either case, altered under the influence of Latin albus (“white”). Not derived from modern Arabic قَطْرَس (qaṭras, “albatross”), which is perhaps borrowed from Spanish.
cf:
https://de.wikipedia.org/wiki/Alcatraz