Squeeze one gently between the fingers and water squirts out from both openings, hence the name. They grow to about the size of a small grape, and are just delighted to be anywhere a continuous current brings them a free fuel supply. In this, they're just like barnacles, which could as easily have grown behind your intake screen. Unfortunately during a mild year like this, they're likely to continue prospering all winter. Sometimes, in very cold years, there's a Mogula die-off and thousands wash up on shorelines of the Middle Chesapeake.
Solutions are few.
You can try to carefully and thoroughly apply bottom paint to the inside of through-hull fittings and the adjacent hoses. I've often wondered why most boats don't have one of these, into which multiple hoses lead for seawater supply. Look at sea squirts and the Bay in a larger context. Both diseases and heavy harvesting over the last one and a quarter centuries have largely removed once-vast oyster reefs that used to be all around the lower Chesapeake. These headwaters include the Eastern Susquehanna and Chemung River watersheds.