Propbndr
LifeTime Supporter
Those are seams in your liner. They, along with any wrinkles, are a catch all for debris. You could be seeing live algae, dead algae or, just plain dirt. When brushed with at pool brush, does it take some scrubbing to remove it or does it just wisk away like dust? If like dust, it is either dead algae or dirt. If it takes some scrubbing, it is live algae.