You see a lot, people showing an algae bloom, then showing their nitrate and phosphate readings as zero or next to zero, but unless your lights are on 24 hours or are very very old tubes if T8/T5 then the readings have to be higher than that. Some algae blooms like diatoms can be due to excess silicates, but unless you've just added new sand or done anything strange that's not likely - not that it looks like diatoms in this case.
Have your phosphates checked with another kit, like a Hanna checker or D-D H2Ocean phosphate kit. It's probably down to overfeeding, 3 small chromis and two small clowns don't eat much at all.
What water changes do you do?