You've got eggs! I am checking on what insect this could be but I've also got two questions; does this pot have a drain hole? and second, is this soil from out of doors or is it potting soil. The timing of transplanting and being out of doors a few weeks ago is telling us that something really liked your plant's soil. If you transplanted using your own garden soil/compost that isn't sterilized you included an insect into your mini ecosystem.
I'll go find some good candidates. If I were you, I'd get a similar sized clay pot with drainage holes and plain potting soil to transplant your plant again. No fertilizer or water holding sponges/gels added to that soil. You won't be able to remove all eggs so don't worry about that for now. I'd keep some of that soil and eggs in a container all by themselves to check out what hatches!
Here is an initial culprit; Oak Lecanium Scale eggs of Oak Lecanium Scale
When you transplant with fresh bagged sterilized potting soil, clean off all dead leaves and debris from the top of your plants. If you are using potting soil then some errant insect found your peace lily and deposited eggs while out of doors. Should not be a problem at all for your lily. btw, do you own a cat?
scale eggs and predators