lichen / liken are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
lichen: (noun) any of several eruptive skin diseases characterized by hard thick lesions grouped together and resembling lichens growing on rocks. (noun) any thallophytic plant of the division Lichenes; occur as crusty patches or bushy growths on tree trunks or rocks or bare ground etc..
liken: NA
lichen: l·AY·k·ah·n
liken: l·AY·k·ah·n
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)