This name is a synonym of Potamogeton × mucronulatus (G.Fisch.) Papch..
The record derives from WCSP
(data supplied on 2012-03-23)
which reports it as a synonym (record 469184) with original publication details: Ber. Bayer. Bot. Ges. 11: 116 1907.
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 Potamogeton pusillus in version 1 of The Plant List.