They're Anthopleura elegantissima. Beautiful little anemones. They are clonal, which means they split down the middle to divide into two daughter anemones. You'll see huge boulders covered in them, even large stretches of beach, and all the individuals are genetically identical. They're white because they probably weren't exposed to much light - in a brighter habitat they would be bright green. They are one of our few local examples of animals that harbor endosymbiotic algae, like most of the tropical cnidarians do.
They will definitely die in a heated tank.
If you picked it up off of a beach and it had life on it that won't survive in your tropical tank (i.e. ANYthing from Washington waters) it's on you to take it back to the beach. It's irresponsible and cruel to cross your fingers and wait for someone with a cold water tank to come get them and save some lives. Add to that it's illegal in Washington to collect anything from the marine environment that is not explicitly called out in the WDFW fishing regulations.