Why is monohydrogen phosphate an incorrect name?
The current version of Nomenclature of Inorganic Chemistry – IUPAC Recommendations 2005 (Red Book) reads as follows:

(…)
(…) The prefix ‘mono’ is, strictly speaking, superfluous and is only needed for emphasizing stoichiometry when discussing compositionally related substances (…).
(…)

Therefore, hydrogenphosphate ($\ce{HPO4^2-}$) may be called monohydrogenphosphate in order to emphasize the difference to dihydrogenphosphate $\ce{H2PO4-}$.
Note that, according to IUPAC Recommendations, the systematic hydrogen name of $\ce{HPO4^2-}$ actually is ‘hydrogen(tetraoxidophosphate)(2−)’; however, ‘hydrogenphosphate’ is an accepted simplified hydrogen name.