Imho the skill doesn't describe it because it doesn't have to. Every enemy in the game with a suppression bar is immune to all status effects while green. To me it's better to explain that you can't slow enemies who are currently immune to slow because their suppression is too high rather than repeat prerequisites on every single ability within the game which just leads to cumbersome tooltips - the suppression system and CC is universal with the exception of the needle toxin, that i'm aware of.

Perhaps they should make the text that states slow be in yellow though for extra clarity.  Might avoid such assumptions that abilities don't have situational criteria.

Well it makes sense for all situational to follow the same criteria - there's probably 100 instances though where text is used without it being colour coded within the game. Might be worth the developers looking to unify it so the system is consistent and easier to understand. 

The actual text of the ability in question though I think is fine, it does have the slow effect added to every shot. It's just it'll only apply to certain enemies. But explaining that shouldn't really be the job of a tool tip - a simple color coding I think would do.