The "Slender" franchise typically focuses on the Slender Man creature and doesn't feature other fictional characters like Shrek.
However, it's possible that some fans have created custom mods or remakes of the Slender game that include Shrek as a character, but these would be unofficial and likely not widely known.
If you're looking for a game with Shrek and a creepy element, there are other games that might fit the bill, although they might not be directly related to the Slender franchise.