Toenail fungus is deep inside the toenail.
Physarum would probably love to eat delicious toenail fungus if it could get at it. But that fungus is down infiltrating the structure of the toenail itself, which is why topical treatments to the nail are (usually) ineffective at curing an infection with this fungus.
Toenails are durable and pretty impervious and the toenail fungus is safe down in there. Physarum is a detrivore and I do not think has the enzymes necessary to penetrate the toenail and get to the fungus. If it did I suspect it would be happy to eat toenail, fungus and all. I would not want to have something like that on my toe.