How to open Moentrol Balancing Spool?

I have the Moentrol shower faucet in my bathroom. After shut off the water main in the house for toilet installation, one of the pressure balancing shower does not work well anymore. Only cold water comes out when another shower was in use. It has no problem when I was the only one using the water. I suspect there might be debris in the balancing spool. However, I can't find any screwdriver that is big enough to open the balancing spool. What tool should I use to open this thing?

TIA

The average person won't have a flatblade large enough for this task. If I remeber correctly, a quarter should work fine for this application...I think a loonie will be too thick.