Reptadeonella insidiosa is a little known encrusting bryozoan. Colonies form white to cream encrustations which are often extensive. In dried material the frontal membrane gives the colony a glistening appearance. Autozooids are elongated, hexagonal or lozenge-shaped with a central depression. They are 0.5-0.6 by 0.24-0.34 mm.
The distribution of R. insidiosa is imprecisely known, but it has been described from the Bay of Biscay, Guernsey, Hastings (Sussex), the Western Approaches to the English Channel and the Gulf of St. Malo. Reptadeonella insidiosa approaches its northern limit at the mouth of the English Channel.
The species is able to colonise hard substrates in offshore waters, and may be common on coarse grounds.