Yep, super glue gel. That's the white stuff on the wood. You can see in the 20L photo above that moss completely covers the glue within a few weeks. That way there's nothing too unsightly. Tying it on with thread also works well.
The 51 white LEDs are 6500K - the 3 blue ones are actinic. Definitely a weak fixture but I suspect it'll be fine for this particular tank.