If it comes off leaves etc. pretty easily when you pull it, but its bases stick to glass & rocks it's a cladophora type - You can try 2-3x overdoses of your tank w/ Excell, best added to the water using a syringe or turkey baster right where the biggest growths are.
If it seldom grows on hard stuff but clings very tenaceously to plant leaves, it's BBA. BBA can be killed on plants by soaking affected areas in ~5% bleach solution w/ gentle rubbing for 20-30 seconds. Sequentially rinse plants in a couple of buckest of treated tap water before returning them to the aquarium. Long term solution: stabilize your CO2 concentration, if you use CO2. I think local additions of excess Excel or small doses of hydrogen peroxide solution will also put a serious dent in it. There are fish & inverts that will help by eating it too.
