You may have a "failure-to-thrive" chick. That means it likely has genetic abnormalities that prevent it from processing its food to get nutrients for growth. This chick may get sick and die. If it survives, it may always be small and may have a weak immune system. Just be forewarned.
You can get Poultry Nutri-drench and give it these vitamins on its food and it might perk up and start growing. If it's not eating, try moistening its crumbles.
Another thing. Your brooder needs to be tossed out. It's entirely inadequate. Chicks that age need a minimum of one square foot of space, and your chicks barely have room to turn around. This could be having an effect on this small chick if it's being bullied away from the food due to overcrowding.
If you have a coop, you could move these chicks in to it right now. If you're still in the process of building it, then rig up a day pen for the chicks and let them spend time outside on the grass or dirt. They need to experience being chickens.