Could be nematodes which will require you to deworm your frog. Best to have a vet confirm and supply you with the necessary medication.

Be aware that they could also be Fungus gnat maggots. They are attracted to moist conditions and will lay eggs in the substrate or on a soiled water dish. They are quite small and worm-like in appearance. Best take the sample to a vet in order to confirm because we cannot do that from this image.