Everything stays clean but the sand bed stays brown

You should have a bit of a color build up as bacteria takes root in your sand.

new tank? could be Diatoms. you can cut back on feeding/light.
Dwarf Cerith Eats Diatoms, Algae, Cyano and Detritus on your tanks substrate and on rocks I added over 300 of these to the 180 and never got a Diatom bloom. Nassarius Vibex Excellent sand stirrer or scavenger they pop out of the sand when they smell food, and when youre feeding the fish.

i agree sounds like diatomes!
cant go wrong with the dwarf cerith they eat all that stuff!
how bigs your tank?

Yeah, like 7-8 months.125g

your tank should be pretty cycled by now lol
what do you keep in their? every when do you do water changes?

alkomist... I take out 10 gallons every week.2 large voltions ,yellow tang,dom damsel,3 stripe dam,blue jaw trigger,mag rabbitfish,half black angel,tomato clown.I use RO water

You might also think about sand sifting stars (depending on what else is in your tank). They, along with my Ceriths, always kept my sand bed white as can be! Unlike some stars (like chocolate chip and bristle) they aren't carnivores and won't endanger your snails, etc.

how long do you keep your lights on for during the day?
i would to you CUC easy fix

Yeah I keep it on for 13 hours a day. I like seeing my buddies

High bio load plus lots of light will make keeping your sand bed white extremely difficult.

My suggestions:
1.) Less light.
2.) Vac it off when you're doing water changes.
3.) Add more flow to suspend the gunk in the middle of the water column
4.) Beefier skimmer/filtration

Essentially reduce the nutrients and light available and the diatoms will die.

Snails and clean up crew probably aren't much of an option given the livestock you have. It'll just be all you can eat buffet for the lions. Your stocking list would have been helpful initially.

I do vac it off,but like 2dayz later it's back.I do have blue leegged crabs.they get left alone.I think i;ll try snails when I get some extra $.