When they are tiny, they tend to be transparent, which allows them to blend into the tank's seascaping and avoid predators. As they grow, the darker ones establish their colors first. The lighter ones go from transparent to white more slowly, and then they begin to get their black marbling or stripes.
Now that they are discovering there are no predators in the tank, they are leaving the safety of the gravel more frequently and venturing higher up in the tank.
Soon, they will be coming to the surface when I feed them.
The sponge that is covering the filter intake is there to protect the babies from being sucked into the filter. As small as they are, they don't stand a chance against the filter's intake. I remove the sponge at least once a week and rinse it off and reinsert the intake back into it. It serves as a biological filter as well.
I purchased the sponge from my pet shop; it was one designed to be inserted into a filter. But I simply took a pair of scissors and created a hole in the center large enough to fit over the intake. I suggest not using filters that are not specifically made for the aquarium industry, as you don't know if any particles exist in them that could harm the fish.