The noise above 200Hz is probably frequency noise from the fiber link which carries the stabilized beam from the corner station (see T0900376).  The broadband floor to the frequency noise is about 1Hz/rtHz, with acoustic peaks from ~50Hz up that reach ~30Hz/rtHz.  This will translate to a flor around 15pm/rtHz in the arm cavity spectrum (see entry 3413, and the ALS wiki page).