Can you just use Guard Duty Runtime Monitoring for ECS?

Amazon GuardDuty ECS Runtime Monitoring can be enabled as one of the Protection Plans available within the GuardDuty service. I would consider enabling other Protection Plans for other services (S3, Lambda, RDS, EKS, EBS Malware Protection) if your workloads are using these services to have a more comprehensive coverage of threat detection.

Please also take note of the prerequisites before you enable GuardDuty Runtime Monitoring