Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Some algae is quite persistent at recovering, even when it seems dead and dried out. Algae spores will find the tank anyway, but you might slow the recovery of the algae by removing the old stuff.
Algae that is dead will rot. This may contribute to the ammonia you need to cycle the tank, if you need to. If your tank/filter is already cycled then the current bacteria population may be able to handle some extra ammonia.
I like to start with a clean slate, and I usually remove old algae. New stuff grows pretty fast, though, so the tank is not algae-free for long.