Yellow leaves on Wax plant most often point to a watering problem. On this plant, the first signs are often yellowing leaves and mushy stems. Check the causes below to find which one fits, then follow its fix.
Overwatering & early root rot. Stop watering. Make sure the pot can actually drain, and wait until the top 4 cm of soil feel dry. If the base is mushy or smells sour, unpot it and trim any brown, soft roots before repotting in fresh mix.
What’s causing it
Too little light
How to tell
The yellowing comes with pale, stretched new growth leaning toward the nearest window. That combination shows that the spot is too dim for Wax plant.
The fix
Move it to a brighter spot, but keep it out of harsh midday sun, which scorches.
Thirst (underwatering)
How to tell
The soil is bone dry, and the yellowing arrives alongside wrinkled waxy leaves rather than soft, mushy growth.
The fix
Water thoroughly (every 7 to 11 days in summer is the usual pace for a Wax plant), then settle into feeling the soil before every watering rather than watering on a fixed day.
Hungry (but rarely the real culprit)
How to tell
Wax plant grows slowly and needs very little feeding, so a uniform pale-yellow fade after many months without any is possible, but check watering and light first: those are far more common on this plant.
The fix
Use a cactus and succulent fertilizer at half the strength on the label, once or twice during the growing season. Overfeeding a slow grower like this does more harm than skipping it.
Simple old age
How to tell
A single bottom leaf slowly yellows while the rest of the plant looks healthy and is pushing new growth.
The fix
Nothing is wrong. Snip the spent leaf off and carry on. Plants retire their oldest leaves.
Prevention
Let it dry out fully between waterings, never let it sit in a saucer of water, and keep it in bright light. Root rot, not thirst, is the real risk on this plant.