Oh shit. Well, Irushia is probably uniquely capable of the old "kill it with fire" approach, but I'm not too optimistic because this seems like a recurring villain set-up.
I think he will not get the ability of that slime to steal/copy skills, instead he will learn a one unique skill since he defeat that slime (if he defeat it anyway)
Cool, time to get rid of arguably a worse threat than the rock dragon.
Also, if he manages to ever get some kind of human form or is otherwise able to live and communicate with Milia, he definitely would've earned it.