This is normal for pretty much any type of wood newly added to a tank. Nothing to worry about. You can leave it or remove it, up to you. It'll likely disappear within a few weeks.
I have it in my turtles tank also, they’ll actually eat it themselves if it gets bad, I believe it to be completely harmless, just doesn’t look good.i once tried removing it but it returned really quickly. solved it by adding an algae eater