I'm looking for a single-worded or a slang term for expressing those marks. I've googled a bit but it did not help. Thanks
Consider imprints (check ODO example below).
1 A mark or outline made by pressing something on to a softer substance
‘A red imprint marked my waist where my stockings' elastic had pressed.’
Artist Justin Alexander Bartels has a photograph series titled Impression based upon these marks.
I would have said "indentations".
...pull an Amanda Lepore and take off all your clothes immediately once you get to the set (optional: bring a loose robe), so you don't have lines to retouch from waistband indentations, sock marks, etc. Suck Less: Where There's a Willam, There's a Way
She pulls the pajama bottoms down an inch, just far enough to expose the indentations left by their elastic waistband. Blow Fly: Scarpetta