Thanks for reaching out to the Apple Support Communities! It sounds like you are having issues with a message that your photo is not downloading from iCloud and I am more than happy to assist! First, check out this article with steps for How to force an app to close on your iPhone, iPad, or iPod touch and close the Photos app. Once that is done, Restart your iPhone, iPad, or iPod touch then check to see if you are still seeing the message when you open a photo.
Also, can you tell me if you see this with all photos or with a specific photo?