I could not find any good scientific information about supplements made from algae and other vegetables, so I cannot answer your question fully. The only information available is from the makers of these products, and I don't consider them reliable.
It is important for people to eat enough calcium in order to keep their bones healthy. Calcium also is needed for other body tasks. How much calcium do you need each day? Before menopause, women should get about 1000 mg daily in their diet. If you have osteoporosis or osteopenia, or are postmenopausal, you probably need about 1200 mg daily.
Osteopenia is a decreased bone density. It is not as serious as osteoporosis, which has an increased risk of fracture.
Measuring calcium levels in the blood does not help us know whether you are eating enough calcium. So you just should try to follow the current recommendations.
Some people can get all their calcium from food sources. Dairy products are a common source of calcium in the American diet. Calcium-rich plants such as spinach, cabbage and kale are another source.
Some foods can slow or block calcium from being absorbed. Certain drugs can cause you to lose calcium. Supplements such as magnesium and iron can interfere with the absorption of calcium. So it can be very difficult to know if you are getting enough calcium. Many people take calcium supplements to make sure.
There are several types of calcium supplements available. The most common is calcium carbonate, which can be from natural sources such as oyster shells or from refined sources. Calcium carbonate is often poorly absorbed, especially on an empty stomach. Also, calcium carbonate from natural sources may contain lead. Chelated calcium supplements, such as calcium citrate, are well absorbed.