Sand Siphon Question

I have a 70g FOWLR tank that has been up and running for about 11 months now, and fish for about 7 months. All my water parameters are good but I have a bad algae problem. I first thought it was the uglies, and it may very well be but I am trying to nail down exactly what it is and how to fix it. I have a GFO reactor on order but I have a quick substrate question.



I got the sand from a LFS. I'm not sure what kind of sand it is or if its crushed coral, but the grain is rather large. The sand always gets this black or green sludge like stuff that grows on its surface like a carpet. Here is a pic of my sand-



Every week I siphon out the stuff, but when I do siphon it and the other detritus, I always end up sucking up a good amount of the sand as well. I haven't minded removing this sand because I want to replace it anyway with a substrate that looks like sand (finer grain). My question is- Is it normal or considered the best practice to remove a little bit of sand every time you clean? I know you cant prevent some sand from being removed, but should you try and remove a certain amount of the substrate ? I have read that you should replace your sand once a year, so if I remove a little bit every cleaning, by the end of the year I wouldn't have much more to remove and replace.



Also once all the sand is removed, what is the best way to replace sand in a living tank? Live sand or dry sand?



Thanks Y'all