Rusticle

Rusticle \RUHS-ti-kl\ , noun;
1. An elongated structure often found on underwater shipwrecks, somewhat like a stalactite or icicle in appearance, consisting of microbial and fungal growths inside a layer of iron oxide

Rusticles on the Titanic (source)

This word was coined in the 1980's (first attested in 1986) because of what researchers found when they found the Titanic. It's based on the form icicle.