Ughh, looks like moisture to me. If you take out all the components to have just the case, you can use almost anything. Try for a start to just use some simple dishwashing soap.
You will need:
1| Some 1"~2" painters brush
2| Cotton wool/kitchen towel with rubbing alcohol
3| Patience and time
You will be able to bring out the system's true color and the spot on your chassis would need some time and care. The case looks like it can slice your fingers(even vegetables) thus the need to be patient.
Case reminds me of the old Thermaltake chassis's...am I right?
Tried my best and after about 1 hour of cleanning with a towel, trying to remove the big chunks of dust with a blade ( it worked realy well until i cut my self) and trying to burn it with the rubbing alcohol those are the results! https://imgur.com/a/iy9Hw ( btw i also broke my front fan, dont know how )