I don't think changes to the coin should be considered. It's no surprise that there are some people with access to a Satoshi-level supply.

With a half million clams, someone can crash the price temporarily. However, as has been mentioned, this isn't the most profitable move.

Compare it to the block reward. With 1 clam reward being produced roughly every minute, there are roughly half a million clams produced every year. If the new big holder decides to sell his supply over the course of a year, it's similar (in terms of supply) to the block reward doubling. In that case it's reasonable for the price to drop by half. If the new big holder decides to sell his supply over the course of a month, it's similar to the block reward increasing to roughly 13 clams per minute. In that case it's reasonable for the price to drop to roughly the 1 mbit level.

Incidentally, while this is obviously bad for the price, it may ultimately cause Clams to climb higher on coinmarketcap.com, since for some reason coinmarketcap.com does not count still-buried clams in the supply.