In most stories and sources, Slenderman is never seen walking normally, and ussually either uses his tentacles to walk or teleports to walk.
So I was thinking...what if he's paralyzed from the waist down? Or just the legs?
Think about it, if he could walk normally, he probably would, esspecially since, in the original mythos at least, he tries to blend in. It also would be more conveniant to walk normally, since teleportation probably requires focus (and it's kind of pointless to teleport one foot in front of yourself, as he often does)
However, he is still able to stand, which actually might be because of leg braces. Franklin D. Roosevelt (aka FDR) used leg braces to stand.If he doesn't have regular leg braces, he could have a pair of tentacles that go down his legs to act as leg braces.
If you think about it...in almost every Slenderman video series, his legs are shown in the same position everytime.