Any ideas on how to remove unknown substance on TRD skid plate?

Perhaps you should dig the rest of whomever you hit out of your undercarriage as well...

 

Its a skid plate, who cares if it gets messed up.

 

In order to remove the unidentified substance, you will need to use an unidentified solvent.

 

The problem with acetone and lacquer thinner is that it is fast evaporating. Try using a good solvent soaked on a rag and let sit to soften it, hopefully it breaks it down enough to remove. You can still use the acetone and lacquer you have just make sure it stays wet for awhile with the rag on it. $.02