The speed of light is the fastest possible achievable velocity in a three-dimensional plane. So, the question is unanswerable. HOWEVER, if it was possible, I believe that to everyone travelling slower than the speed of light, you wouldn't have a shadow. But from your perspective, it would look like you have a shadow that goes forever behind you.
The Principle of Relativity states that a vehicle moving at constant speed, disregarding wind resistance if the contents are not inclosed, has the same physics as one not moving at all. Einstein himself also worked out that time distortedly slows down in a moving vehicle, but only from the perspective of a person outside the vehicle. One of his brainy colleagues then proposed this problem - if you were on a train moving at the speed of light, wouldn't you be able to just walk forward in the train to travel faster than the speed of light? Yes, however... in order for a person to see something, light photons have to bounce from their eyes, to the object, and then back again. The light can go back to the object, but it can't catch back up to your eyes. Therefore, if you looked behind you, it would look as if you were in a room with no lights on.
Back to the posed shadow problem, seeing as though you do and don't have a shadow at the same time, but whether or not you do is dependent on your perspective, it's sort of like the Schrodinger's Cat paradox. Look it up if you don't know it.
So, in conclusion, yes you should still be scared of Slenderman even if you move faster than the speed of light. Oh, wait, no that's not the theory. Sorry, I'll try again - in conclusion, whether or not you have a shadow or not is dependent on your perspective. At least, that's the reckoning of my 13-year-old brain, I'm probably wrong in some way. :C
Good theory, me gusta. :)