The default slender mod hud can be really messy for lower resolutions
So there are 2 commands here you can run to solve it for the time being.

--Type the following commands in console--

--Camera hud--
slender_hudtype 2

--Scoreboard hud-- Also works with slender_hudtype 1
slender_scoreboardtype 2

These commands will set the hud to the Stop it, Slender hud
Anyway hope this helps a little.