The commensally living faltworms pose no threat to the shrimp, they just sit on the shell and do not disturb the animals
Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed.
Scutariella are flatworms that are specialised shrimp ectosymbionts. You usually get them with shrimp that are already infested (often with wild-caught shrimp). Scutariella have been proven to live on shrimp from East Asia, India, Sri Lanka, but also from Central Europe and the Balkans. This whiteish worm belongs to the Scutariellidae family. It grows to approximately 2 mm in length and has two lobular tentacles on the end of its head and a sucker at the bottom end. It is often found in the gill cavity of shrimps, but also on the basis of their antennae and the rostrum.
If Scutariella sit in the gill cavity you can only see them in more or less transparent shrimp like Yellow Fire, Blue or White Pearl. If these shrimp are infested you can see small white spots on the inside of the outer gill cavity wall. Very rarely you can also see their spindle-shaped body. Scutariella are easier to find when they sit on the basis of the antennae and around the rostrum of a shrimp. There you can see stick-like or spindle-shaped worms, which wave the upper part of their body in the water. These worms can crawl spanner-like, but sit in one place most of the time. /
taken from Breeders and Keepers Vol III – Scutariella, Vorticella and Stentor by Andrea Früh