In most cases, cloud is used generically as a common noun or adjective, and should start with an initial lowercase letter.
Examples
This application is available as a cloud service.
Use the web administration portal to provision phone numbers in the cloud environment.
With the rise in popularity of cloud computing, people are concerned about access to cloud-hosted data in the event of a natural disaster.
Exception
Use an initial capital letter when Cloud is used in a product or solution name, or at the beginning of a sentence. For example, Google Cloud is a name, so Cloud starts with an initial capital letter.
