Are these flatworms?

[Olympius]

Noticed this scurrying around my glass recently. Are they flatworms and are they cause for concern? I have had a string of bad luck lateIy and I am currently treating the tank for Bryopsis and Dinos.

 

[Dan_P]

Yes, flatworms. Not sure that all flatworms are an issue though.

 

[vetteguy53081]

Noticed this scurrying around my glass recently. Are they flatworms and are they cause for concern? I have had a string of bad luck lateIy and I am currently treating the tank for Bryopsis and Dinos.

Yes acoel and not as dangerous as having red planaria but can be an issue when in numbers. Whay happens is, that congregate at the base of coral thereby blocking needed light for coral to produce Zooxanthellae which is their source for light and energy/growth.
If only a couple, simply siphon with a 3/8" tubing and discard. If many- Siphon and add a wrasse that will eat them such as yellow coris-6 line-lunare- melanurus or even a springieri damsel.
Also look daily for next 2 weeks for precense of eggs which are really tiny and scrape into net and remove.