Summarize:

Ground seaweed is used to add an oceanic taste to many Asian and coastal recipes around the world. Opt to use spirulina powder instead of seaweed to make the oceanic flavors much more potent and to add a boost to the nutritional value of your meal.  While they don't taste exactly the same, as long as seaweed isn't the main component of the dish, you won't be able to tell a difference. Raw spirulina can be used instead of raw seaweed as well in just about anything except sushi, as the algae doesn't exactly wrap around other ingredients very well. Spinach and spirulina have wildly different flavors, but they have the same color. If you want to "trick" your kids into eating spirulina for its nutritional benefits, substitute it for spinach in your recipes. Plus, it has more than 2000 percent more iron than spinach, bringing a boost to your iron intake each day. While you can't substitute spirulina for cooked spinach, it works well if the spinach component of your dish is not the main element. Test it out with a few small recipes before you commit to making a meal out with it. One of the most common ways for people to take spirulina is to simply mix 1 tsp (5 g) with a full glass of water. You can use a tiny amount of spirulina in place of food coloring for a more natural option. Pour tiny amounts of spirulina while stirring into the recipe you want to turn green.  Most recipes call for at least 1 tsp (5 g) of spirulina to get its full nutritional value, but if you are just changing the color of a dish, opt to go for a 1/4 tsp (1.25 g) at a time so you don't add any extra oceanic flavors. If you add too much, the recipe will turn a deep, dark green. Make sure to use a pinch of powder so you don't overwhelm your guests with a surprising algae taste.
Substitute spirulina for ground seaweed for a more powerful flavor. Replace spinach in your recipes with spirulina. Use small amounts of spirulina in place of green food coloring.