This is a very poetic line that needs some explication to understand its significance. At this point, everyone but Ralph, Piggy, and Samneric are with Jack at Castle Rock. Those who are with Jack have essentially abandoned all sense of order and civilization. Therefore, the conch, as a symbol of power and a means of establishing order, is now useless. However, Piggy still intends to use the conch. Being partially blind from having his glasses taken and weaker than many of the other boys, the conch is all Piggy has left. Even Ralph can not protect him when they confront Jack.
Piggy has been a cerebral but physically vulnerable character from the start. Now, with only the useless conch, he is completely vulnerable. So, there are two connections between Piggy and the conch: order/civilization and vulnerability. This quote symbolizes the vulnerability of both Piggy and the conch and it foreshadows the destruction of both. A single drop of water drops from Piggy onto the curve of the delicate conch, flashing like a star, or like a light going out: vulnerable and ephemeral. When Roger kills Piggy, the conch is smashed into a thousand pieces. Piggy falls forty feet onto a rock and his body is washed into the water, where the conch originally came from.
Then the sea breathed again in a long, slow sigh, the water boiled white and pink over the rock; and when it went, sucking back again, the body of Piggy was gone.
This quote and the first quote cited in the question establish a connection between Piggy and the conch. The progression of these symbolic connections are sequential: order, loss of order, vulnerability, and death.