I've worked retail in The Netherlands for quite a while. I've had customers point out things that needed my attention. Of course, there's different ways this can be done and some are nicer than others, but in general, people are likely to overthink this: I'm working the store, I'm there to handle situations from 'the toilet ran out of toilet paper' to 'someone's trying to steal'.
In general, it's nicest if people ask me to fix the problem, or just point out a potential problem, instead of commanding me to immediately fix a problem. I've always preferred remarks like 'Could you (send someone to) refill the toilet paper if you have the time' over 'You must refill the toilet paper'. Similarly, it was more helpful to get 'I saw that person put a small item in their pocket, perhaps you should check it out' than 'you should call the cops on that thief'.
In order to avoid 'telling someone how to do their job' it's better to err on the side of asking, or making sure to convey that you understand the situation is potentially a problem, instead of presenting it as definitely a problem that they should fix.
Instead of telling an employee 'someone over here is not wearing their mask' (which would sound like there's definitely a problem you want them to fix right now), ask them if they are aware that there is someone that isn't wearing their mask.
You could pre-fix that with a remark about how you fear the store might get into trouble, something like "I've heard stores could get into trouble if customers aren't wearing masks, are you aware there's someone not wearing their mask in the pancake isle?' would work. A question also leaves open the options for the employee to reassure you that the store won't be in trouble, or that they are aware of the problem already (but can't handle it for some reason).
One last note: When you've told the employee, don't look like you expect them to drop everything and handle the situation right at that moment, themselves. In cases where other people are involved (like shoplifters or potentially angry customers), an employee might just tell you 'they'll handle it' without immediately looking like they'll jump into action. In cases like that, sometimes all the employee has to do is alert a supervisor/store manager/boss. Just thank them and move on with your shopping.