Unlikely any vegan would have any moral qualms about algae, given that they’re not animals. Maybe you were thinking of oysters/clams/bivalves?

What vegans would not be ok with algae?

Yeah I took that for a while, and did slightly improve things. Only problem I had at the time was those were super expensive in the UK. Maybe that's changed now

I think they meant their vegan diet didn't naturally have Omega 3 in it if they didn't take supplements; not that they couldn't take the supplements once they realised it.

Or fish oil