I'm not worried about the brush algae "spreading" because algae cells are in the water anyway. Whether or not the algae multiplies (and the type of algae that grows) depends on whether the conditions are there for the various types of algae to grow or not.

The conditions in the tank (that I have been putting the plants into for the fish to eat the algae) are not good for algae growth - light, CO2, plant fertiliser levels are in decent enough balance that algae can't take hold in the tank. Or at least grow to levels that the siamese algae eater and other algae eaters can't keep in check.

It's no use being paranoid about algae because it is as present in water as dust is in the air.