Welcome to the forum!
Likely iron. Do you have a skimmer? If so, put some polyfill in the basket. It should capture some of it.
Have you added the proper chemicals at start up or just added chlorine without regard to level added? High FC will bring iron out of solution.
How are you testing your pool water chemistry?
I suggest you read through
Pool Care Basics - Trouble Free Pool and even look at a few of our videos
TFP-TV - Trouble Free Pool