Aliens. Definitely aliens.

[d2mini]

Was about to clean my glass tonite and was like hmmmm.... I wonder what those tiny little white specs really look like?
So I grabbed my macro lens and I was not expecting this! Little starfish-shaped critters.
These are literally just a white spec to the naked eye.

 

[Sean Clark]

Leave them be, in a few weeks you will have yourself a pyramid.

 

[IKD]

Look like hydroids / mini jellies. Harmless and go away on their own in time.

 

[AydenLincoln]

Hydroids very common in a new tank they will die-off as your tank matures.