This name is a synonym of Symphyotrichum spathulatum (Lindl.) G.L.Nesom.
The record derives from TICA
(data supplied on 2012-02-11)
which reports it as a synonym (record 60B72BD1-3FED-4450-A73A-BDF5339DAE46).
This name record (from TICA) was not included in version 1 of The Plant List. It is possible the same name was included from a different data source, to check this you can search for Aster occidentalis in version 1 of The Plant List.