This name is a synonym of Rumex usambarensis (Dammer) Dammer.
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 100372074) with original publication details: Pflanzenw. Ost-Afrikas C: 169 1895.
This name record (from Tropicos) 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 Rumex nervosus in version 1 of The Plant List.