help i got grey mould eating my outdoor plant!!????

hey guys im new to this an this is my first grow i have 2 plants in my greenhouse that are growing fine i think,but i took out the biggest one as it started to get grey furry mould eating away at it??!! i have cut off most of the affected areas,but it keeps coming back,my freinds have said its root rot?? i dunno what to do?? any help will be greatly appreciated!!???View attachment 1161090

im not really sure what that is. ive nver had that kind of problem on my stems. it could be root rot. if it is you should transpplant. have you tryed looking at your roots yet to check them out?