SonicsDC25: another alternative you can do, is ditch the algae project, and use repashy foods, it's this powdered stuff that turns into a jello-like consistency when you add boiling water to it.
Because of that, you can be as creative as you want. You can put the mixture into a mold so that it is in certain shapes, or place it on a baking sheet/tupperware, and let it jello-ize, then cut into cubes.
OR
"paint" some of the mixture onto the rocks, and let that sit and jello-ize.... once the repashy has turned into jello, you've got rocks with edible goo on it, and your fish will go CRAZY!!!
The best part? The food stays stable for up to 24-48 hours in your tank, so that means, it won't cloud up, or foul your water.
Another great thing? You can "paint" the rocks ahead of time, and store them in the fridge for up to two weeks, or in the freezer (jello form) for up to 6 months! Powdered form will stay safe for...well, pretty much forever (well, I mean, you'll most likely use it all up before it "expires".)
It definitely beats waiting to grow algae on rocks, that's for sure