Should I remove the 1 plant effected by powdery mildew?

If you have a humidifier, remove it, your plants will be fine, and in fact, cannabis will produce more trichomes in a arid/dry atmosphere. I would be removing the infected plant though, just put it outside in a sunny but dry spot if you can, otherwise there is a great risk of the mildew spreading. A great risk.

Thanks @StoneyBird420 :) I will definitely look into this as well. After each grow I make sure to disinfect and clean. I always like to have a fresh and clean start to ensure nothing is left over from the previous grow. Fingers crossed I'll be able to have some smokable stuff by the end.

Yes IMO remove it. Actually the entire tent needs to be disinfected (lights, fans, stands any equipment and all of the plants should be treated (from my understanding) there are treatments for PM that are safe to use during flower. https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.thehealingcanna.com/growroom-problems%2520powdery%2520mildew&ved=2ahUKEwj5pLKd5LnzAhXOpXIEHdGsCQ8QFnoECDAQAQ&usg=AOvVaw3jqI2tJm95wdTVXiLYjb6h This is a great resource for how to treat and what's safe to use during flower. Good luck chief! Nice growing too btw!

@Frm11001 Thank you very much for the info! I had heard that during the flowering stage that it's a good idea to have your RH @ ABOUT 40% - 42%... It said that it's not a bad idea to have your RH at this level as this will increase the resin or trichome production.. something about how if there is a lower humidity level that the formation of trikes increases... I don't know how true that is but I will do the homogenized milk treatment on all plants to include wiping the bottom of the leaves. Ill also monitor the RH during the lights off period and will update you on what I find. A little about my 1 that's close to harvest.. I had her outdoors all summer until the weather got to be wet & humid here (typical Washington lol). So I brought her inside. She was doing great and then she started showing the powdery mildew. I'm not sure if that is one of the contributors? I had a friend do the same thing, although he had a different strain & set up/location. Did fine outside and after he brought her in, he had P.M. issues as well.

Based on my experience in the last grow, you will need to keep applying homogenized milk whenever you see it necessary until the harvest is finished. I applied it once a week, and even then, I couldn't completely combat it. Your humidity seems low, which I find strange that you have this issue. You should check during the "night" if the humidity increases a lot; if so, try to lower it during that period with more ventilation. That's what will make the difference along with the homogenized milk. Also, the milk prevents powdery mildew spores from attacking the other plants. I had 4 plants very close to each other, and only 2 had that issue. It also depends on the health of the plant, meaning they can stay together in my opinion, but significantly improve the ventilation 24/7 and don't stop using the homogenized milk. Apply it well all over, even underneath the leaves.

Thank you @gmslave for the response! I just did a treatment of the 60/40 milk and water about 1 1/2-2 weeks ago. Maybe safe to do another application. Is there any point during flower I should stop doing this or is it safe to do until the end?

You can manage the PM with 40 parts homogenized milk to 60 parts water and spray at sundown or lights out. Repeat as necessary.