The 2nd to last toolbar button on the Schema Browser gives you a legend of all of the icons. The ! icon for materialized views indicates "needs compile". This information comes from the COMPILE_STATE of DBA_MVIEWS. You can read about that in Oracle docs, here: https://docs.oracle.com/database/121/REFRN/GUID-8B9432B5-6B66-411A-936E-590D9D7671E9.htm#REFRN20139
-John