I have no clue… a challenge, are they harmless?

[mrnicebug]

A challenge for the advanced guys. I have no clue…. What is this here critter and should I be worried? Pics under 6500k daylight

 

[randomfishdude]

Hydroids?

 

[ISpeakForTheSeas]

Yeah, looks like digitate hydroids to me - they're generally considered bad:

The suggestions I’ve heard include picking them out with tweezers, starving them out (a few different methods for this, but the one quoted below seems to be the most prevalent in use), and using dwarf angefish/certain wrasses to eat them.

Looks like a colony of digitate Hydroids. You will need to starve them simply by directing water flow towards them, not at them and they will soon starve

 

[EeyoreIsMySpiritAnimal]

Colonial hydroids!
Yes, get them out of your tank by any means necessary!