Plz I have a reef aquarium and would like to know if there is any coral/anemone safe sea slugs which are visually appealing and only eat algae/uneaten food.
You might look into the lettuce sea slugs of the genus Elysia. They're algae-eaters, although they don't eat all that much. Rather, they absorb the chloroplasts of the algae, and then bask in the light to take in nutrient through the photosynthesis of those chloroplasts. Keep in mind that if (when?) they try to crawl across your powerheads ... they're toast. They've been known to get sucked into powerheads and shredded.
There are plenty of good-looking sandsifting gobies, they should have a larger tank (how big is yours?), as they'll be feeding on the biomass in your sand, and can clean out a smaller tank pretty quickly. Algae blennies like the starry, lawnmower, lined or tailspot are great for helping with algae on the rock.
None of them is a substitute for good maintenance and husbandry, BTW . . .
Edit & P.S. to the above post - Welcome to Reef2Reef, Alexytman! I hope you find that this place entertains, teaches, and is worthy of many repeated visits - just as I have!