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I notice that there are dark stones, which can increase the amount of heat in the area. Unfortunately, compensating with more water tends to lead to infections. Shading the area will be the only safe way to mitigate the heat without overwatering.

A good rule of thumb is to let the soil dry, completely, down to about 2 or 3 inches between watering. Since there are stones on top, it will not be possible to do this by looking at the soil. It may require a meter to measure, accurately, the soil moisture content.

Here is an article that will help:

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm

Should you start to notice widespread spotting, or sudden widespread death, then I would suspect Verticillium wilt or Phytopthora.

https://www.gardeningknowhow.com/plant-problems/disease/verticillium-wilt-treatment.htm

https://www.gardeningknowhow.com/plant-problems/disease/what-is-phytophthora.htm

In this particular case, Aptenia should be treated as a xeriscape, or desert landscape plant. This means, NO watering. They do not tolerate wet soils, and would rather be left on its own.