[ehfeng]

Assuming you pulled it straight from the sea, it'd probably be salty and slimy.

The internals of cells are mostly water, protein, and DNA, which don't really taste like anything to our tongues. Most sugars are stored as glycogen, so you wouldn't get much sweetness. There also wouldn't be much texture, as it's lacking any sort of bone or shell.

[learnstats2]

You are describing an extremely nutrient-rich food with no immediately apparent defence against predation. To the extent that it is difficult to understand how this could exist in the wild.

I suspect that this alga has some interesting and perhaps surprising defences, in line with its relatives. (And, that these may well impact on its taste)

For example, spirulina is said to be spiral because this allows it to spontaneously flee from approaching water fleas, with no active energy use: primarily depending on the laws of physics.

[amfsn]

If we leave glycogen in our mouth for seconds/minutes, would the acids in our mouth decompose it into glucose which would taste sweet? Yes, I have no knowledge about chemistry.

[tryptophan]

Nope. There is no acid in the mouth, and acid doesn't break down carbohydrates anyways, its all enzymes in the intestine.

Amylase, the thing in the mouth, breaks down starches, like in rice or bread. Glycogen is different from these though, as it has different linkages, and can't be broken down by amylase.