I believe the exec node does not allow mustache template.
You could create the command prior and append it with msg.payload.
there are many ways to do this of course.
Here is one example using the template node.
[{"id":"1194e34726faa84f","type":"inject","z":"e1ef5728e9206b4a","name":"","props":[{"p":"payload"},{"p":"topic","v":"true","vt":"bool"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"10","payloadType":"str","x":210,"y":620,"wires":[["8c67ed5a9667154f"]]},{"id":"8c67ed5a9667154f","type":"template","z":"e1ef5728e9206b4a","name":"","field":"payload","fieldType":"msg","format":"handlebars","syntax":"mustache","template":"set_power {{{payload}}} -s powered {{#topic}}1{{/topic}}{{^topic}}0{{/topic}}","output":"str","x":380,"y":640,"wires":[["74ee6f08590cce85","0d37b44178b16b05"]]},{"id":"daf93507509ec81f","type":"inject","z":"e1ef5728e9206b4a","name":"","props":[{"p":"payload"},{"p":"topic","v":"false","vt":"bool"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"10","payloadType":"str","x":210,"y":680,"wires":[["8c67ed5a9667154f"]]},{"id":"74ee6f08590cce85","type":"debug","z":"e1ef5728e9206b4a","name":"debug 2472","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":570,"y":620,"wires":[]},{"id":"0d37b44178b16b05","type":"exec","z":"e1ef5728e9206b4a","command":"","addpay":"payload","append":"","useSpawn":"false","timer":"","winHide":false,"oldrc":false,"name":"","x":550,"y":660,"wires":[[],[],[]]}]
You would not need the change node to convert the booleans doing it this way.