I don´t think it is slang, nor are some compund adjectives, such as ever-lasting, left-handed, open-minded, goal-focused...

It is not slang. It's perfectly acceptable even in formal writing. Double-checking with a quick Google search, I find, picking only examples from recognized authors: "The space of sky above us was the colour of ever-changing violet and towards it the lamps of the street lifted their feeble lanterns."--James Joyce "I forgot that there were fields, woods, rivers, seas, an ever-changing sky outside the steam-dimmed lattice of this sick chamber..."--Charlotte Bronte "The Ever-Changing Sky: A Guide to the Celestial Sphere" (1996 book title) "The Ever-changing Union: An Introduction to the History, Institutions and Decision-making Processes of the European Union" (another book title) "The Sea, in storm or calm, Heaven's ever-changing Shadow, spread below, Have its deaf waves not heard my agony?"--Percy Shelley