What is decaying?

The new cannons decay after a short amount of time..
Can't you just pick it up and replace it?

 

http://runescape.wikia.com/wiki/Multicannon#Decay

You can just pick it up though.

If it decays fully then you repair it at the new Artisans Workshop.

To be honest I don't understand fully as it also says that decayed cannons can be reobtained from Nulodion. If you just use it as before, picking up and reassembling when you get the warning then you should be fine.

 

If you leave your cannon set up for an extended amount of time, it will explode/disappear. You can get a new one from the cannon dude for free. It was like this with the old ones too