I have an issue that is spreading!!

About 2 weeks ago i noticed that i had a mold issue on one of my plants. The mold was on the stem and was starting to travel. This week i was getting ready to harvest another plant and AGAIN the plant is brown. Except this time nothing flies off the bud when i blow on it like the last moldy plant. After i finished clipping the plant i cut the stem into about 5 sections and i noticed the stem was hollow. I have grown in this room before and never had a mold issue. Could this be a bad CO2 tank??? I'm confused on what is causing this. I looked through my plants closer and now two others are showing symptoms. It almost looks like heat damage. But im guessing the same thing is happening, the stems are hollow and the plants are dying from starvation. Could the buds be brown just from die off? The only reason i am leaning towards mold is because some of the plant is green and the rest is*&&^%%$* BROWN. Is there anything i can do???

Damn..............................

Most pot plants stems are hollow, and you can make a mild vinegar solution and spray on the mold to get rid of it.

Thank you for the reply and i will try the mild vinegar spray. I wonder what started the outbreak.