Is the brown dust dead algae, or do you think it's live?
Is it 'coming back' or just settling back down, in roughly the same spots?
Mustard algae can be brown, but it's more often yellowish.
The fact that you added DE and didn't see anything back in the pool is good - that means your filter is catching fine particles like it should.
But, you'll need to post your other test results, especially your stabilizer level. Meanwhile, "orange" is a good chlorine level to maintain.