Not an expert but it appears to be on an older leaf. Thinking a little frost damage or maybe sunburn on a baby leaf. The rest of the plant looks great. I would just trim that leaf.

I just noticed the same thing on my heucherella Solar Power yesterday, just a couple of the older but not old leaves. Leaves that were grown this spring but just further down on the plant. SP lost pretty much all of its leaves this winter so they are all new this spring. I'm wondering if maybe it is cold or frost damage? Other than those couple of leaves, which I removed, the rest of the plant seems fine.

Karen