Are those nudibranchs?
Look like sponges to me.
Edit: Pineapple sponges on second look.
yup thats a white sponge. they are harmless but very ugly. most commonly found in the sump and places with lower flow. very common in a new aquarium.
They will go away over time. Common to have an explosion of them for a bit then they just naturally die off. They are a sign of a healthy tank IMO.