My Mandevilla plant is blooming beautifully but today I noticed small white splotches on ALL the petals. They are not raised splotches, just irregular 1/8" to 1/4" splotches.
Problem With Mandevilla Petals
Certified GKH Gardening Expert
From this image it appears to be perhaps just damage from water. If it is appearing only on the flowers and not the leaves, this is what I would say it is and nothing more serious.
Inspect the rest of the plant.
Some issues of pest and fungus can be treated with Neem Oil, which works as both an insecticide and fungicide.
The links I listed have more information on possibly pest and disease. Stay proactive!