On Thursday morning all was right with the Lagoon Pond. The water was clear, blue-green crystal, by all appearances the very picture of estuarine health. Just a day before, the water was clouded by an unsightly yellowish-brown fog from the head to the mouth of the Lagoon. It was an explosion of prorocentrum, an algae, and the largest one that Martha’s Vineyard Shellfish Group director Rick Karney has ever seen.