I'm not sure which project this is, but it looks like it's some sort of clay or plaster of paris
Hello. That’s a great question. I’m not sure my best answer would be a vinyl caulk.
Looks like drywall compound to me.
It's most likely unsanded grout. Same stuff that is put down between pieces of floor tiles. Some people fill in tile mosaic pots with drywall compound or Plaster of Paris but it does hold up as well as grout compound, especially if you will be using the pot outside.
It has the characteristics of vinyl caulk to me. Have you tried removing some of it to see what the total texture might be? Does it feel powdery/rubbery/paste like? How does it respond to getting wet?