I don't think I'm going to be able to complete this as I've never had the algae in the sea do you think this is a glitch

you have to keep fishing the pond and the algae will appear.

Ummtalhah is right. I got algae pretty quickly in my game, but it took my 8-y/o daughter forever to get it. She never had her villagers fish, but that's all I had them do. The problem with me was that I couldn't fish for quite a while and was struggling with food until I was able to find the algae-eating fish.

in one of my new villages, i only had level 1 farming so they all fished, and when their food bin reached about 1000, green algae apeared!!! now i'm down to 10 coconuts!!!!!

and in my other village, they are going for the coconuts, and the ocean is still clear for fishing!