Yes, scraping it with a razor blade does spead the spores. Try using a siphon while you're scraping to help lessen the spore count.
Algae grows solely on the glass for a few reasons. First, you may not have a clean-up crew that can tackle the glass easily (nerites are great at this!) Ceriths, hermit crabs, etc prefer the rocks. Same goes for the sea hare. Next, if the tank is old, small pits in the glass can actually cause silicates to leach (yes, glass DOES leach silicates...just in very very very small amounts very slowly...so yes, your tank is dissolving away over time.) These small pits are also great places for spores to collect and grow. Flow and lighting may also be more conducive for algae on the glass.