That does look suspiciously like cancer. There is that microscope video that provides fairly compelling evidence that at least some cases are caused by a bacterium that eats into the plastic, which means the age of the affected toy is irrelevant as long as it's the right kind of plastic.
When I find a spot I soak the pony in a dilute bleach solution overnight, never had it spread after that. I'd be nervous about not taking the head off for the soak though, it is quite the pain to remove G4 heads. On custom bait I slit the back of the neck between the rows of hair plugs to take the head off, then cut around the edge of the neck plug as carefully as possible. To reassemble I glue the neck plug back in with E6000, put the head back on, then glue the back of the neck shut again with some more E6000. It stays flexible and clear, holds well but opens up easily with a little touch of the xacto if you need to get in there again.