This name is a synonym of Plectranthus monostachyus subsp. lateriticola (A.Chev.) B.J.Pollard.
The record derives from WCSP
(data supplied on 2012-03-23)
which reports it as a synonym (record 454156) with original publication details: J. Linn. Soc., Bot. 58: 271 1962.
This name record (from WCSP) 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 Solenostemon monostachyus in version 1 of The Plant List.