It Looks exactly like hydroid jellyfish!!!! The tank is set up for a few months but all my live rocks and sand were transfer from a 3 years old tank.

Is it harmful should I remove them?

Far as I know from reading they dieoff once there food runs out mine where gone after 5 months give or take

I had them too for a while. Showed up on the glass for a few months and then they just disappeared. Wish the aiptasia would have been as polite.