This man's request for replacement discharge documents are filed (in the WW1 Service Records, WO 363) with the records of a Charles MARTIN.
MARTIN & MORRISON may be the same man, but no mention is made of an alias.
Perhaps this is a simple mis-file:

Farquhar MORRISON:
Filed a request for replacement docs on 19 June 1939, aged 59 years;
Home address: Oldmill Hospital, Aberdeen.
Service (first engagement, so served in WW1 - possibly as MARTIN?): Jan 1897-1909, as Private 5727 Cameronian Scottish Rifles.

Anc****y, MAR, pages 136201-136214 (many blank)