If you want to get rid of the crushed coral, don't do it slowly, you will only stir up detris that has settled. How long has your tank been running and how deep is the crushed coral? Could cause some serious issues depending on how deep it is.There seems to cobwebs growing on my sand and crushed coral bottom wondering should Ijust vacuum it up or is it some kinda good for my tank. I know Ishould get rid of the crushed coral I've been doing it slowly. Wanna get a daggonet gobie should Ileave half of crushed coral in there for coipods to reproduce?
I would pull all the LR out, catch all the fish, drain the tank, remove the crushed coral all at once, replace with sand you want, replace LR and fish with at least 50% of the water that was in the tank when you drained it and leave it be after that.
As for the cob webs, most likely red slime algae or diatoms.