My rock has gathered this strange substance which kind of looks like algae but it’s got bits through it. Starting to come on my whole rock. Just done a water change and noticed it
My guess would be some kind of turf algae that's collecting detritus. If it feels soft but can't be easily pulled off and is hard to scrap off with a sharp metal edge it's almost certainly a turf algae. One of the short spine urchins are the first thing I would get to help control it. You can also use steel straws to scrape and remove it. H2O2 in a syrenge and applied against the base or "holdfasts" on the rock should kill it but it will be a tedious job.