Could be a few different things. It could be precipitated carbonate, it could be bacterial buildup, or it could be broken down algae/food/detritus.
What is your Ca
, testing at? This can help determine if it is precipitate. Most likely, if you are not supplementing any of these, then it is detritus. This is why canisters are often not recommended for sw
tanks - detritus can get trapped and build up, and cause nitrate issues. What are your nitrates testing at?