Newbie opening existing pool.

I have an above ground pool. The cover was not on properly so leaves and other debris have fallen in. It is green and with buildup on the bottom of the pool. I'm not even sure how man gallons it holds/how big it is. I took out most of the leaves.

It has a hayward pro lx sand filter and electronically controlled pump. After reading some things online I decided I should backwash first. The pressure gauge was at 31 psi. Once I moved the multivalve port to backwash, I started the pump. There's no air in the pump while backwashing and the pressure goes to 40 psi. I look at the sight glass and see some sand in it but relatively clear. After about 7-8 mins I stopped backwashing. Set it to rinse for about 30 secs then filter and notice that there seems to be some air in the pump. It increases to 50 psi after about 5mins so I quickly turn it off. Also, when I look at the return water, I notice that its not clean and has debris.

So what could be the problem that I'm having?

Also the local pool retailer sold me some liquid shock to pour in while its filtering. Haven't done that yet. So what else would I need? Chlorine? ph balancer?

Are you sure of those pressure readings?

Most AG pool pumps don't have the capability to produce those pressures no matter what. What pump make / model do you have? If those pressures ARE correct, you've got some severe circulation or filter problems.

Regardless, get started scooping out everything you can. (Have a leaf net?) It's not practical to try to burn out leaves and debris with chemicals.