5 Oct 2018
Nullhide Ferox's activated ability can be activated only while it's on the battlefield. Players can't, for example, activate it to try to stop its replacement effect from putting it onto the battlefield.
5 Oct 2018
Once Nullhide Ferox has lost its activated ability that makes it lose its abilities, that ability can't be activated again until the turn's over.
5 Oct 2018
Once Nullhide Ferox's activated ability has resolved, it can gain new abilities. It won't lose those abilities.