my case front mesh is really dusty and it seems there are stains that won't go with simple cleaning so is it safe to use water to clean it and let it dry or maybe this will cause damage to it and affects air flow ?
Assuming you can remove it, take it to the sink and clean it, or run it through the dishwasher. Make sure it's completely dry before re-installing. I do this to the dust filters on several of my cases around once a year or so. I typically use a leaf blower about every six weeks or so inbetween.