Yellowish stuff on pool walls

Hello. I a, new to the forum and me to pool ownership . Never wanted to own a pool but I have Rheumatoid Arthritis and need pool therapy 3 times a week at $50 co pay each. Pucharchasedca house with a pool in dec 2016 . I know nothinging about pool maintenance . I have an unground kidney shaped pool with vinyl liner , 18000 volume , 6 ft at the deepest end , 1 skimmer, 3 return jets , 1 drain , sand filter amd intellichor salt cell .

We have some sandy yellowish stuff in the pool , when brushed in the shallow end it looks like dust in the water , I vacuum and in 2 days it's back . Also have what's looks to be the same on the wall in the deep end but the brush doesn't affect it . I had a pool company come out and they said the algae is impregnated in the liner and they couldn't help . Is that a real thing ? I need help clearing what ever is happening . What do I need to provide to get your help