Sounds like an exceedingly poor idea to have a terminal=false line in the.desktop, have you all actually tried this?
Testing your .desktop here has 2 effects -
The icon in the launcher has no control over the mplayer instance
The cpu use is abnormally high, 99% of one cpu
Here I do use a mplayer .desktop that does control the instance & cpu is low as expected - an Ex. (excuse my icon, a green apple is ok for me
This is used both from the r. click context & as a DnD in the launcher