Too much light, too much nutrients and poor plant growth can all lead to algae. I’d try to siphon the bottom as well as you can without distributing any plants, think about raising your light (it’s easy to have too much in a shallow tank) and remove any dead plant matter you may find. Are the plants growing well? Algae can still happen if they are, but if she aren’t, getting your plants happy would be the first step, to help them outcompete. I remove some green thread algae every week and just ordered a phosphate test kit to see if that’s a factor.