realized the above is needlessly complicated:

(defn states-in-ns [ns-sym]
    (->> (vals @@#'mount.core/meta-state)
         (map :var)
         (filter #(= (namespace %) (ns-name *ns*))))

your find-all-states idea is better, since it makes it less "internal":

(filter
  #(string/starts-with? % "#'app.whatever")
  (mount/find-all-states))