I'm guessing it's Genetics, many Hoyas have it.

Don't know what it's called technically or botanically. Here, we sometimes refer to it as Speckling. IME it's common on Pubicalyxes & some Obovatas.

Let's see what others have to say about it.

You might consider posting this in the hoya forum. A picture of what you are referring to might help as well. If you are referring to "splash", I think there is some mystery/debate about why some hoyas have and get "splash" versus not.