He was then assigned as an artillery observer in Flanders as well as participating in minesweeping operations in Friedrichsort. In April 1917, Greiser volunteered for service in the Naval Flying Corps where he initially served as an observer with SEE I and II and then with Küstenfliegerstaffel I and II. From August, 1917 to August, 1918, he was assigned as a naval aviator to Marine Schutzstaffel I. During this time, he was transferred to Seeflugstation Flandern II (Ostende) and he later flew with the Seefrontstaffel and MFJ IV.