Plants are not perfect and never will be Lower leaves often get spots, turn yellow, wither and die It's not even an issue. Keep doing what you're doing. LITFA. Too many cause worse problems trying to fix nonexistent "issues". J
Never water a MJ plant unless you can lift the container it's growing in and it feels as light as it did when first loaded with fresh soil. They hate having their roots sit in moist conditions all the time and allowing them to dry properly forces the roots to search out the entire container for food and water, which gets you a larger root ball and a stronger plant in the end. This looks like you spilled something on a leaf...no biggie. You judge the overall health of the plant by what the new growth looks like. If it's that rich blue/green color that we associate with a healthy plant, you're good. If you start to see the color wash out and fade to a more yellow/green, your plant might be getting a bit hungry. But there is nothing wrong with this plant and it's old growth anyway. So no big deal. Sounds like you need to do some reading up on how to tend an indoor MJ plant. Most only concentrate on the setup they will put together to grow with and devote little time to learning how this plant grows and how it should be taken care of. You need to get the very basics down like when and how to water, proper grow media, the importance of lighting strength/quality for each individual plant, space to grow, etc. A dead plant gets you no harvest. Best of luck . TWW