Is it time to cook

I have two tanks up and running my first tank 72 Gal bow I have had for over 5 years now and my 220Gal that has been up for a year. the 220 I started from scratch But my first tank was an existing set up that I bought and I have no idea how many years it was up and running before I had it. The past few months I have had to travel a lot for work and my tank cleaning has suffered. My one year old tank looks fine. The older tank is full of green allege on all the rocks. I keep trying to scape as much as I can off and do water changes but it does not seem to help much.

My thought is to put some fresh dry rock in give it some time to populate then start to cycle out my old rock and cook it then put it back. Over the past year I have pulled almost all the coral out of the smaller tank as I was converting it into a small predator tank. right now the only live stock is a porcupine puffer and snow flake ell