Yesterday I watered it and other than some yellow leaves it was fine. Today this! The inside seems to be healthy still. I don’t see any bugs.
I was just reading about Plumeria. I didn't know they were a type of succulent (which is how I ended up reading about them). I have several succulents and the one thing I've learned, since getting them, is they don't like to be over watered, preferably left a little dry. If something has yellow leaves, I have watered too frequently. They love lots of sun. Here's the article I read:
I am not an expert on Plumerias, but this does not look diseased, looks more like a watering condition. If you suspect disease, you could cut off a section and take to your local nursery and have them evaluate what is going on.
I would try watering less. Let the soil dry between watering.
Here's a post that might help - https://www.floridacolorsplumeria.com/how-to-spot-nutrient-deficiency-symptoms/#:~:text=Plumeria%20Nutrient%20Deficiencies,as%20foliage%20discoloration%20or%20distortion.
Too much water?
Take a cutting to your local nursery.