I know slenderman has many different writers and isn't very consistent when it comes to that. So i'm going to be using some of the powers that have been given to him over the years. This slenderman is very powerful jsut to let you know.
POWERS & ABILITIES:
Immortality
Rapid regeneration
Immune to disease
Immune to radiation
Immune to fire
Reality warping
Teleporation
Slender Walking, which gives him a form of omnipresence
Slender Sickness, which gives people looking at him a terrible disease
Time manipulation and time travel
Cloning
Radiation emission
Dream manipulation
Telepathy
Hypnosis and brainwashing
Creation of physical illusions
Creation of hallucinations
Creation of Proxies, and by extension Life Manipulation
Intangibility
Tentacle Generation, as well as the ability to create even more appendages. Did I mention that they can crack space-time?
Voice mimicry
Matter manipulation
Pyrokinesis
Electrokinesis
Umbrakinesis (darkness manipulation)
Water manipulation
Energy blasts
Earthquakes and weather manipulation
Telekinesis
Technokinesis, He can control technology but it also gets distorted just from his prescence.
Dimensional manipulation
Probability and causality manipulation
And finally, his final trump card, Dimensional Bleeding. What's Dimensional Bleeding? It's an event that spreads tremors throughout entire universes and dimensions, destroying chunks of space-time everywhere and forcing them to be "filled in", thus causing universes to literally collide with one another, possibly even causing their destruction. Fun!