Keep the castle if you like it, you're the one looking at it every day.
And you can add more rock if you want, but it's not required, there are fish-only tanks that have no LR
. As long as you provide some hiding places for the fish to feel safe, it's fine. Couple questions:
how big is the tank?
How many fish in it?
How much do you feed and how often?
What are your levels:nitrate, phosphate, etc?
How long are lights on for every day?
Do you have any PHs and how's many?
Is it slimy and comes off easily? If not, it could still be diatoms.