how do i get rid of algae on sand?

[Capt_kulafu]

hello fellas i'm running 30g saltwater for 5months my problem is how do i get rid of algae on sand? every 2 weeks 20% water change and by using siphon tube.

my filter is HOB w/ protein skimmer

check my picture >>>> on my last message

livestock:
1 lawnmower blenny
1 domino damsel
1 blue yellow tail damsel
1 blue damsel
1 yellow damsel
1 black clownfish
4 or more hermit crab
2 or more snails