Mr Croall was most recently chief operating officer of Global Advanced Metals and prior to that, general manager operations at Lynas Corporation.
He was the resident manager at Lake Johnston from 2006 to 2007.
Robert Boston has joined the Poseidon leadership team as general manager corporate, commercial and legal, after having served as a consultant since 2012.
Mr Boston has previously worked at BHP Billiton Nickel West and Rio Tinto (exploration).
Poseidon has made former Millennium Minerals and Integra Mining senior executive Vanessa Hughes manager people and culture.