Alright, I'm not sure exactly what to say but basically Zalgo just met slender while with Sanny and he's tugging at Slender's tentacles because he's never seen tentacles like that and he's extremely curious. Slender doesn't understand why he's being touched and in my headcannon tentacles are very sensitive so it probably feels weird to him. And Sanny and Splendor I would imagine wouldn't like each other much at first despite their younger brothers getting together....
also.... for some reason I couldn't come up with a good title today
Zalgo (c) creepypasta
Valhorah (sanny) (C)
Splendor (c) creepypasta
Slender (c) victor surge