I've had the same problem...the algae is the last puzzle I need to solve and it's never appeared.

My villagers just constantly water the crops..which never appear unless I'm actually watching it, and they never fish.

The only thing that's keeping them alive at the moment is the stew that heals, or they'd all be dead.

I think my problem is I solved the crop puzzle before the algea appeared and it's caused a glitch or something.