Pink Mold In Pool

[ltpowel]

My pool is inground 14X28 and about 10,000 gal. The pool is clear as a bell, last week I saw a little pink mole around the seams, I brush it and added 1 gal. of shock. about a week later their's some pink mole on the seams again. but not as much as before. What is causing this pink mole and how can I prevent it.
Thanks, Larry
free chlorine=3
combine chlorine-0
chlorine=5
ph=7.5
alk=100
calcuim hardness=270
cyanuric acid=70
temp of pool water 84 F