What is this symbol on the Ghost? It's located on the side (possibly only the left) of the front loading bay.
Probably a model symbol
According to the schematics from Star Wars Rebels: The Visual Guide:
The Aurabesh text above the logo seems to read:
(Note that the cresh is backwards in my version, for some reason).
Or, in English, line-by-line:
That is, of course, the ship from which the Ghost was originally modified.
This answer might suggest that it is the logo of the Corellian Engineering Corporation, since that is the text that appears below the symbol in the image presented there. However, this logo does does not appear on the Millenium Falcon or other ships manufactured by the CEC, so this seems unlikely.
It's the symbol of the Ghost
This symbol first appeared on a pin given to people who attended the Star Wars: Rebels panel at Star Wars Celebration Europe 2013
This Blog post and a post in this thread suggest that the people who got the pins knew that the symbol was that of the Ghost. Seeing as how the show didn't air until 2014, someone official probably told them what the symbol was.
Just to note: the pin included a hidden url that lead to an early version of the picture in @Adamant 's post.
Furthermore, the symbol has 2 elements that lend itself to being uniquely the Ghost:
Two green triangular tear drop shapes, and then 2 smaller green lines below that resemble Hera's leku (head tails). Remember, the Ghost is Hera's ship.
The shape of the symbol is reminiscent of the ship. (Octagon)
All this together leads me to believe that this is the symbol of the Ghost itself (and not of the VCX-100)
I used ColonialChrome's ghost reference in the picture above.