Magic menagerie wrote:

I hear green algae is ok, though too much isn't. Our tank seems to have too
much brown gunk/algae. I scrubbed it off three days ago, and now it's back...
What is it?

Brown algae is diatoms. These are fed from excessive phosphates, silicates or
nitrates.
increasing the lighting strength and time can encourage green algae to grow
instead.
Nothing to worry about though, they are perfectly safe.