Yes, when you requested it. FEMA pays the requesting entity, which is legally responsible for the work, not the agency that helped. Mutual aid is eligible for emergency protective measures, emergency utility restoration, building code and floodplain work, and grant management.
Source: PAPPG Table 16.
If you don’t normally pay mutual aid partners, don’t agree to pay them just because FEMA might reimburse it. FEMA checks consistency with past practice.