


"On the Rocks" plant has new leaves with red spots
I'm not sure the species of plant. It was advertised as "On the Rocks" and I've had it for around 4 years.
It sits on a rock that is glued to a shallow dish filled with stones. The instructions said to never let the stones fully dry out, which I've been mostly faithful to over the years. The plant has grown steadily the entire time, with no issues.
I noticed on it's 3 more recent new leaves, that there are red patches on the leaves, with the amount on each leaf increasing with each one (as in, first leave had a couple, 2nd a bit more, and this newest baby leaf has the most).
I tried to research causes, but the top three causes were edema due to too much water/soil being waterlogged, pests like spidermites, or changing the light source.
Issue is...I don't get how it could be any of those?
The plants roots sit in water. There is no soil. I have changed nothing in how often I water it. The instructions even said to never let it totally dry out. I have done that a few times over the years, but never for prolonged periods.
There are no pests or spidermites. I know pests and I unfortunately know spidermites. There's none.
The light source is unchanged. It has sat on this shelf beside an East-facing window (I live in Southern Australia) for almost 2 years now. The only change to the light are normal seasonal changes. Nothing else.
What is wrong with my plant?