I'm not sure what is causing the growth, but I would recommend taking it to a vet.

Growths like these are often difficult to diagnose without testing/biopsy.

From
Diseases of Amphibians - Christine L. Densmore and David Earl Green:

(1) There are many causes of lumps and bumps among amphibians, and a diagnosis of neoplasm based solely on gross clinical appearance is often inaccurate. (2) Granulomatous inflammatory responses, encysted parasites, and infections by mesomycetozoa are common causes of nonneoplastic tumors, which historically have been mistaken for neoplasms with some regularity (Green and Harshbarger 2001)

Not saying it is a neoplasm as viral / bacterial, etc infections can also cause growths.
I'm no expert on amphibians but if I had to guess it looks like it could be a granulomatous inflammatory response to something perhaps.

Others may provide better insight...but I would take it to a vet to see what it is so you can treat properly.