Put a lid on it, and you'll have no more algae, other wise, yes this is
normal with any filter that light can get to. You could sort of consider it
a veggie filter.... but it could also clog and channel your water flow.

It is a veggie filter, so can't put a cover on it.
Maybe #2 won't have any plants, just a cover.

The drain is a 2" pipe. I glance at the pond quite frequently
throughout the day, so if that drain does get clogged, I will notice
it quickly.