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.

Según mi experiencia en el ultimo cultivo vas a tener que seguir aplicando leche homogeneizada siempre que lo veas necesario hasta terminar la cosecha, yo lo aplicaba una vez por semana y aun asi no logre combatirlo completamente, tu humedad parece baja, lo cual me parece raro que tengas este inconveniente, deberas ver durante la "noche" si la humedad aumenta mucho, si es así, intenta bajarla durante ese periodo con mayor ventilación, es lo que marcara la diferencia junto con la leche homogeneizada, también la leche impide que las esporas de mildiu polvoriento ataquen a las otras plantas, yo tenia 4 plantas muy cerca una de otras y solo 2 tuvieron ese inconveniente, también depende de la salud de la planta, es decir que pueden seguir juntas a mi criterio, pero mejora considerablemente la ventilación 24/7 y no dejes de usar la leche homogeneizada, aplicala bien por todos lados, incluso debajo de las hojas

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.