Beginner Gardening: HELP!!!!!!!my cacti are turning into cardbord, 1 by cactibobo

cactibobo wrote:
As you can see my beluverd cacti has a strange cardbord substance on it and i dont know what it is!! and if you look at the outher pics there is also some black stuff on the bits that are coming off and a little on the eges of the main stem if this helps i have always warted them about 1-2 a month and thay have been sitting on my south facing window sill and also i have a plastic greenhouse if i have to use it tell me you probaly have notest a completly diffront cacti with the same cardbord stuff on the bace i have had this one for 2-3 years and the problem only started 2-4 months after cristmas same with the first one and the black stuff on the first one only started when i put this cacti outside(i think) and weve had a bit of rain recently so it could be overwaterring but as i said im not sure i started when i put it outside
MANY THANKS TO ALL THAT HELP ME
P.S THE BLACK STUFF AND THE CARDBORS STUFF DONT FELL MUTCH DIFFRONT FROM ACTAL CACTI IT SELF
MANT THANKS CACTIBOBO