Cute little phals you got there!
I love the minis too.
The brown spot surrounded by the yellow margin looks like bacterial or fungal infection.
Or mechanical damage followed by secondary infection.
Whatever it is, when you see something like that on the leaf (or leaves), trim the affected area off.
Be sure to cut about one inch into the green healthy area to make sure you get all the "bad" part out.
Whatever you use to cut the part of that leaf off, use something really sharp so it cuts well to minimize any unwanted damage to the leaf while cutting.
Do Not use the cutting tool (whatever you use) on other plant in case you spread something to them.
It shouldn't be something serious and trimming that part off should take care of it for now.