"Mass Flow Rate" is a measure of Mass flux. The common symbol is $\dot{m}$ (pronounced $m-dot$), although sometimes $\mu$ is used. The SI units are $kg s-1$.
Applicable units are those of quantitykind:MassFlowRate
http://en.wikipedia.org/wiki/Mass_flow_rate
https://cdd.iec.ch/cdd/iec61987/iec61987.nsf/ListsOfUnitsAllVersions/0112-2---62720%23UAD107
http://dbpedia.org/resource/Mass_flow_rate
$q_m = \frac{dm}{dt}$, where $m$ is mass and $t$ is time.
http://www.iso.org/iso/catalogue_detail?csnumber=31889