No antiboitics unless you know it is an infection.
If the chick has cocci, get a coccodistant because they are protozoa.
It does look reddish, but does not look like bloody poo. Cocci will present with lethargic chicks before bloody poo happens, so if the chicks are lethargic, have been exposed to soil, then consider cocci. If they are in a brooder, jumping as usually, just one poo that way and not lethargic, it's probably just fine.
Take up the suspect poo and any vet will be able to run a test on the fecal sample "fecal float" and be able to tell you if they have cocci or worms, AND what kind so you can get the right meds to treat.
I had a 4 week old accidentally get 2 sunflower seeds and his pooh that looked like that for about a day. I think it was just too rough for him because at that point he just had sand for grit and it was a bit harsh on his system.