Do you have a quarantine/hospital tank? Thats a good size for that. Dont know about rock, I would suspect you would need adequate lighting and skimmer and good water parameters, but parameters would fluctuate with small tank.
I think that would work well, get a good light and a pretty good filter and let it go, my brothers tank is a 10 and it's growing like crazy, just don't have fish in it and the stuff will thrive, i'd not even clean the glass, let it grow
A friend of mine in Florida uses a spare 20H tank to "seed" rock on his front porch. It gets about 3 hours direct light every day in the summer. Just don't expect a ton of crustaceans to survive if you try this outdoor method, since it gets hot. Most algae don't seem to mind.