I agree with Ben. Let's treat the algae first, then we can deal with the FC levels.
The amps and volts are in the proper range, other than when the Check System light comes on and the voltage and amperage drops.

My best guest is that the cell is worn out on one side, moreso than the other side.

To confirm, when you get the Check System light, lift the cell cord off and move it over to the other holes in the cell cord.
The cell cord has three holes, two of which are used with the SC-36 cell.
(That is, if I number the holes on the cell cord as 1, 2, 3 and the prongs on the cell A, B. Assume 1 is on A and 2 is on B, lift it off and place 2 on A and 3 on B.)
Make sure when you do this, there is nothing in the third hole (there should be a red plastic insert cap). By doing this, you've reversed polarity on the cell.
IF the message goes away, then the cell is worn out on one side and not the other.
IF the message stays, then it's possible that the Power Module is damaged and not reversing properly.