String interpolations are only allowed in node and attribute contents contents, so instead of using the shortcut class syntax, you need to specify an explicit attribute: 

div
    - foreach(device; mDVC.mDevices)
        div.hauto-device(class="device-#{device.data.eco}-#{device.data.ecoDevType}")

The class attribute and the .hauto-device shortcut will be automatically merged into a single class="" attribute.

Perfect, this works. Thank you. Vibe.d is awesome