Nematodes are brownish in color like heather said and are typically not harmful to frogs, but will decimate your micro fauna population. Only way to get rid of them is to tear the tank apart, toss the soil in the trash, bleach dip the plants, and bleach wash the tank.
Not sure if that is what you have or not. I would be willing to bet all the money in the world they aren't spring tails. Springtails are smaller and very clearly not work looking.