First, a bit of background: Home is under two years old, rear patio is a limestone/calcareous, fairly porous, white in color rock. Appears to be a more or less flat rock, locally known as 'flagstone'. Patio installed with above rock, drains well, standing water not an issue. Problem: The installer pressure washed with hot water the rock prior to sealing. Subsequently, the rock has shown a great deal of dis-colorization, some algae and the patio marks very easily. This has appeared about eighteen months after sealing. Homeowner has witnessed the prep work and sealing by patio contractor. However, contractor will not return to correct/address problem. Activity to date: I pressure washed with a non heating machine, applied both chlorinated bleach and driveway cleaner (separate applications) and still the patio is a discolored mess. Question: Given the above information, how can I return the rock to a uniform appearance? What kind of sealer/sealing agent should I apply? Provided those whom answer this post, please include a supplier/website for any chemicals/equipment needed. Thank you in advance.