Fist make sure no dirt / grit is on the surface. It never hurts to use a soft camel’s hair brush. Start brushing at the center and stroke outward. I use ordinary Windex but most are squeamish, and use purchased lens cleaning fluid. Just so you will know, I have used Windex on cameras, binoculars, telescopes, microscopes etc. for more than 50 years and never once harmed any lens. But a word of caution, use fluids sparingly, you don’t want any liquid to creep under the rim retainer. If the spot doesn’t come off using Windex or purchased lens cleaner, I apply injection grade alcohol diluted 50% with distilled water using a fresh laundered, well used “T” shirt. Be gentle, start in the center with a moist cloth and work in a circular pattern to the edge. My experience is: Optical coated glass is more durable than people think. I also know that alcohol is used to clean during initial assembly. Keep in mind that distilled water works also and it evaporates without residue. Without residue is the key phrase when it comes to lens clearing. I often make my own lens cleaning fluid. I use distilled water and vodka 50:50. No vodka, which is Ethel Alcohol, I buy Ever Clear at the liquor store, it is 100% Ethel Alcohol. I cut it 25% with 75% distilled water. Works for me.