Rock with strange debris

[cammuir9]

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

Any help thanks

 

[Timfish]

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.