Brake cleaner is very volatile (may still be ether based like it used to be). It will completely evaporate away, leaving no residue.
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I still think it's possible that the shock is leaking. Follow that "grease spot" up under the upper shock shroud tube with a flashlight. Does it go into the protected area? If it does, it's likely a leak.

 

Well, I have to take my car in for the 60k checkup very soon, so I'll just have them check it out when I take it in. Thanks for everyone's input!

 

I would try to clean the area the best you can, then after a few hours/days look at it again, if the mark returns I would suggest that your strut is shot or soon to be. Not to worry, if it is blow it is an easy fix..