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There are several causes for yellowing foliage, but the number one reason is stress-underwatering, nutrient deficiency, shock, pests, etc. Pinpointing the cause for this stress is important in treating the plant. This article should provide some ideas: https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm

Could be it is time to repot it and also make sure it is getting plenty of sunlight. They do tend to drop older foliage as new foliage comes in, kind of the cycle of life thing for them. It sounds like you are watching the watering which can cause such problems. Also be sure the pot you plant it in has a drainage hole or drainage holes. Also when watering it, do not let it sit in the bottom saucer full of water for more than a half hour. Letting it sit in a saucer full of water will cause root problems the same as overwatering does.

Here is a link to a website with more information for you. Congratulations to you too! You have a seed pod formed up!! Nice!!

Link: http://mgonline.com/articles/desertrose.aspx

And a link to an article on using the seeds:

https://www.gardeningknowhow.com/ornamental/flowers/desert-rose/desert-rose-propagation.htm

Usually once they're dry on the plant, you can remove them. This article may also help: https://www.gardeningknowhow.com/ornamental/flowers/fgen/harvesting-garden-seeds.htm

Go to the following link and read the third article.
This article should help: http://www.siamadenium.com/articles.html

If the plant is only 6" tall, it is still considered a seedling. It will branch as it ages.