Ericaceous formula for acid-loving plants. Feed your ericaceous plants the nutrients they need to grow twice as big (versus unfed plants)!
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Yes, those leaves show chlorosis. The product Forsoothe recommends contains the chelated iron. That means it has a molecule that binds iron ions contained inside. Do you know in what kind of potting mix it was originally in? Since it was slip potted, if that pH is too high, the iron supplementation will help with the chlorosis. Not doing anything to address the chlorosis may not solve that issue because it was slip potted and the roots are still in the old soil. Probably a good decision to slip pot it. But the cause of the chlorosis is likely the old soil mix.
I don't follow the idea that doing something that is generally good for a plant, namely fertilzing, will kick-start a struggling plant. I believe in creating a low stress environment for a struggling plant. And removing the causes as for why it is doing bad.