If you experience slime, the cycle is
1) Dark slime
2) Lighter / greener slime
3) Turf algae begins
I wouldn’t quite say it’s a sign of an immature scrubber (although I can see a justification in saying that) it’s - true statement sometimes
I would phrase it as a sign of a scrubber still in, or that reverted back to, the early stages of nutrient removal/reduction in a a tank
Slime algae grows and is very efficient at absorbing nutrients. Super efficient. Darker, almost black slime is more nutrient filled than lighter / greener slime. They both also prevent turf algae from gripping on a surface, and prevents it from developing.
But the slime is happening for a reason. And the turf is not growing for a reason. The tank has excess nutrients but the slime is growing faster and claiming it’s property first. It needs to be harvested more frequently and the screen needs to be cleaned so that turf roots have something to grab onto. Eventually the slime should stop growing and turf should develop. But if it doesn’t, the scrubber is still functioning fine, slime algae is working the same way, it’s just uglier and needs to be harvested more often