It sounds me like you have a couple of issues going on at the same time animals almost invariably bite for one of two reasons – to attack food and in self-defence.
Now, if pretty much the only time the frog has seen a human hand inside its cage and in front of it, is when it is fed, it will natural come to associate the two events (like Pavlov’s dog). This is the most like explanation for that behaviour.
Frogs are not territorial in as much as they do not defend an area where they live. However, if they feel threatened and are unable to run away or hide, they will, if sufficiently stressed, attack the source of that threat. Males are also known to be quite vocal at such times, letting out a loud squeal rather than the usual mating call. Water skinks are predators of small frogs and large frogs are predators of small water skinks. It would be at all surprising if each was uncomfortable in the company of the other, especially if they each had near misses with the other when younger. I think what you are seeing between the two is a defensive reaction. If this is the case then both animals will be stressed by being kept in close proximity and should definitely be separated. You should be able to see a distinct change in behaviour within a short time following separation if this is the case.
People do keep water skinks and frogs in the same enclosure but the size is normally substantially bigger, both frogs and lizards are adult and the frogs have plenty of arboreal room with branches and foliage into which to retreat. Even then, it is still questionable if it is a good idea.
Blue
