The easiest and most reliable way to tell the difference between a macro and a ghost is seeing how long the forearms are basically, as Demeter32 already mentions.
For illustration purposes, here are two picture (both pictures are from generic online google images), first image (top) is a ghost shrimp and second image (bottom) is a macro, you can see the slight diferences between the two is hard to tell but for the differing forearms length with longer arms and larger claw on the macro.
There are other very slight body colouration and saddle colour differences between macros and ghosts etc but even those are not infalliable to tell the difference between the two species, forearm length and claw is the most dependable way in my opinion.
Hard to tell when you just have one shrimp on its own, I know, but if you compare that one in the picture you provided with one of the other 19 ghost shrimps you have, thats probably your best bet. Once you know what to look for, it becomes easier.
One other possibility if it IS different from the other ghost shrimps and differs from macros is that it may be an Amano shrimp, all three species, macro, ghost and amano look extremely similar when they are young juveniles just to confuse matters a little more, sorry!
I'd suggest you google amano, ghost and macro shrimps for further images online to try to determine what looks closest to what you have in your tank.
