The filter pressure is dropping because there is a reduction in flow rate. Most likely a partial blockage in the suction line or something is stuck in the pump impeller.
If the elevation drop kinked the line anywhere, that may have caused it.
Also, you can first try to flush out the suction line backwards through the pump basket using a hose and rag to seal the line.
If that is ok, open the pump basket remove the basket and see if you can see anything stuck in the impeller. You may have to remove the motor to fully clear it.
Also, to kill the algae, you need to shock the pool. Chlorine got too low for your CYA and algae took hold. Filtering only removes dead algae but does not kill it.
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