Looks like mustard algae.
There are two types of mustard algae. One type is fairly common. It sticks to walls and floors. It's bright yellow and covers large areas.
The other type is rare. It it usually brown with some yellow. It looks and acts like sand. It doesn't stick to walls or floors. It mostly collects on the bottom.
It can seem similar to dead algae in how it acts, but the key difference is the color. Dead algae will be mostly white or gray with no color. Colors, like brown, yellow etc indicate live algae.
I think that you have the second type.
The rare type of mustard is very chlorine resistant and does not use up a lot of chlorine, especially when there is only small amounts of the algae. So, you can pass an overnight chlorine loss test.
To get rid of it, you do a full SLAM. Afterwards, I recommend maintaining the fc at 15% to 20% for a week and then 10 to 15% for two weeks.
The yellow is due to beta-carotene, which is an anti-oxidant. It helps protect the algae from oxidizers like chlorine.
Here is an example from another poster.
Spot forms daily?? What to do ??? W/ pics
