What miss her

This looks like and older, lower leaf. I'd wait for it to progress before doing anything too drastic. interveinal chlorosis is what this is called - paling that starts between the veins. More than one thing can cause this and Mg-deficiency is definitely one of them (but not Ca). However, sometimes older leaves just deteriorate. sometimes it's from the excess moisture being so close to the substrate or just age. They mottle, and get a little fugly at times. So, i'd see how this progresses, compare to a leaf symtpom chart to rule out the multiple possibilities based on how and where on the plant it progresses. if this is isolated and minimal, be patient, or you might accidentally throw kerosene on a tiny ember and turn a small issue into a bigger problem.

Generally this is down to calmag. Ad m0use suggests. Make sure you match the ph range for optimal condition in whatever sub you using.

You are the issue.

Calcium, maybe Magnesium, maybe PH,