mrblur

i wonder what it is. it is also occur on the body of my skimmer. i suspect that it is an algae. however my phosphate and nitrate is 0. i change water 20% weekly. i turn on my MH lighting from 18:00 till 24:00 i wonder why it is still arise. so these are my question.. 1. if it is an algae, which type of algae is this? 2. how do I get rid of it? 3. can i use Turbo snails or Blenny to help cleaning on this alge?or is there a btter way ? 4. is it because of the water flow in my tank? 5. why is it occure on the body of my skimmer?

now i am doing 20% water change once a week. i am using Coral life salts. is it too much ? and after the water change, do i need to drop anything to make it stable anyway my tank is 1500 L thanks for your advise