You shouldn't, but you can. All repeating rifles work better with brass. But will you HURT your FAL with steel cased ammo? Well, you will likely shorten the life of the chamber a bit. But if that means your rifle only lasts 9,000 rounds instead of 10,000, but you save a lot of money, is it worth it? Only you can say.
But when you consider all modern western militaries choose brass cases over steel, you should ask why. It comes down to cost cutting. The communist bloc had no choice but to economize and pinch pennies. But just look at the war in Ukraine to see where that policy got them.
If you do use steel cased ammo in your FAL (or any weapon), just make sure it is in PERFECT condition. Steel cased ammo rusts so easily. And firing rusty ammo in any gun will indeed cause extra wear and even reliability issues. And if you are putting ammo away for long term storage, choose brass. Or if you MUST get steel, buy only in sealed tins and keep them away from moisture.