Oh, they’ve been in there for around 3 months, would it take that long for them to begin leaching?
Have you tested your water for ammonia, nitrate, nitrite, hardness? Are your fish healthy and active? Do you do regular water changes? If you're water's good & the fish have been healthy in that tank for a while, it's probably a type of algae. I've seen "white algae" once in a guy's breeding tank; it presented no problem (if that's what yours is). As for the rocks, they do look like limestone in the photos. Limestone rocks will contribute to water full of calcium. Where I live, the water is already very hard. I deal with this by replacing evaporate w/reverse osmosis water and by my weekly water changes. If I were in your shoes, I'd take out the limestone rocks, test my water & keep up w/water changes. Half a century of fishkeeping has taught me that barring the introduction of new fish w/out quarantine, all problems in the past came from water quality (and bad water due to overfeeding). I don't mind algae in my tanks (I like the feeling of peering through a window into an underwater world) & just keep the glass clean so I can watch the fish. The tank where I've got hundreds of 2-week old 3-spot gourami fry is full of algae, which seems to make for healthy fish. PS: You might want to scrape the "algae" off your filter; that part does look like calcium deposits in the photos, although it may not be.