I had two types of the acoel flatworms, a smaller red type and a one that was a bit larger and brown. I just treated my tank to eliminate the brown one, though the red population is likely eliminated too.
The red ones were detritivores, and the closest they came to a coral was to consume any dead tissue - like maggots. They were 99% around or in detritus. Their populations would wax and wane with how dirty my tank was i.e. their food source. Watching their behavior, it was obvious they were harmless. I had these for several years and they never hurt any corals.
A couple years back, I acquired a different variety, a larger brown type. These flatworms are 99% on corals, so this type is obviously parasitic. Recently, I have something in my tank killing corals, so I treated my tank to finally kill these off. Previously, for me the risk of killing everything else in my tank was not worth it. I only had several dozen of these 'worms.' However once I did treat, it went well with no noticeable collateral damage, thus far.
I suspect from your physical and behavioral description, the red ones you have are harmless detritivores.
Our outdoor gardens have many types of worms, mostly good and sometimes occasionally bad, but to most gardeners killing them all "just to be safe," would be considered silly. In my opinion, the same is true regarding the variety of worms/planaria/acoels in our reef tanks i.e. it really depends on where their living.
