That substrate seems very wet. While spiderlings do require a bit more humidity, and moister substrate, having that much wet substrate is going to cause issues. Also, that type of vial does not allow any cross ventilation.
I would suggest that you move them to a deli cup, or a soft plastic vial with holes you can punch with a thumb tack down the sides, and have about half the amount of substrate in it. That will give them some room to dig, but not make it a perfect place for mould to develop. If you don't have something small enough to be a water dish, you can trickle a bit of water down the sides dampening the substrate a bit, or put a piece of sphagnum moss inside that you can wet down.
@VanessaS Thank you very much! I had just wet it down and honestly probably ended up wetting it a little bit too much. I have planned on moving all my slings to those deli cups pretty soon, as I've heard good things about using them vs. the pill bottles. These bottles just came with them when I ordered them, so I've been using them for a short while.