I've been noticing these really small creatures (or plants, I'm unsure) in my tank recently. They measure less than 1cm in length and I've only seen them at the bottom of my tank; some attached to the gravel and some to the glass. They have tentacle-like appendages on the end of their stem(?) and seem to drift with the with the water movement rather than on their own accord.
My tank is a planted tropical freshwater.
What is this plant/creature in my tank?
Any additional information on whether or not these are harmful to my fish would be appreciated.