It's milfoil - an invasive plant species. Left unchecked it can choke a lake, robbing it of oxygen.
Now it's in Lake Washington - and it's very hard to get rid of. Some of the most popular swimming spots on the Eastside are affected.
Now, crews with Bellevue Parks are working with neighbors to attack the creeping plant. But to do it, they will have to shut down three parks for a day this week - putting a crimp in summer plans.
"This is a very popular beach and my kids come here a couple times a week," says Jeremy Crossland.
On Monday, Meydenbauer Beach will close for 24 hours starting at 7 a.m.
Clyde Beach Park will close down at the same time, not reopening until 7 a.m. Tuesday.
On Wednesday, Newcastle Beach Park will be off limits to swimming all day.
While swimming areas will be closed, the rest of the parks will stay open.
To control the milfoil, Bellevue Parks officials say crews will use an herbicide.
"We're kind of an organic type of family so that's a concern," says Bill Ingram.
"They're treating it for just one day, is it safe the next day, apparently," adds Patty Ingram.
Parents are already making other plans.
"I guess Wednesday will be at our house in the pool in the backyard or something," says Crossland.
Officials say a one-day closure will help preserve the lake for years.