“Cloud” is one of those everyday English terms whose legal meaning is totally unrelated to its common one. Indeed, in legal contexts “cloud” has absolutely nothing to do with nubes, but rather is basically a synonym of “defect” as used in the expression “cloud on title.” Black’s 6th explains that cloud on title is an “outstanding claim or encumbrance which, if valid, would affect or impair the title of a particular estate,” while Black’s 8th simplifies the definition describing it as a “defect or potential defect in the owner’s title to a piece of land arising from a claim or encumbrance.” Aspects that could be considered a “cloud on title” include either (1) liens, mortgages, judgments and tax levies (etc.) on the property in question, or (2) actual defects in the title deed itself.
“Cloud on title” is a difficult expression to render in Spanish. The Cabanellas-Hoague EN-ES dictionary simply defines the concept as factor que incide negativamente sobre la certeza o validez de un título inmobiliario, sea por referirse a la existencia y transmisión del derecho previsto en ese título o los gravámenes o cargas respecto de tal derecho, without offering a Spanish translation. Google Translate and DeepL’s nube sobre el título obviously won’t work. The definitional renderings imperfección del título and título insuficiente (Alcaraz-Hughes) give an idea of the meaning, but perhaps don’t ring true in legal Spanish. And the same may perhaps be said of the defecto de título that often appears in Internet sources.
Adding to this difficulty is the fact that in this context the meanings of “title” and título may not actually be equivalent concepts. In that regard, “title” often refers to a document evidencing ownership (an already-registered “title deed”—title to my house; title to my car), while título denotes the documents evidencing different types of transactions that a rightsholder may apply to have recorded on the the Spanish Registro de la Propiedad. Thus, registrable rights (títulos inscribibles) include not only ownership of property (dominio; propiedad), but also any other of the rights in property (derechos reales) that one may seek to register (hipotecas, usufructo, servidumbres, etc.)
So, getting back to “cloud on title,” if the reference is to liens, mortgages, judgments or tax levies, perhaps “cloud” can be viewed here simply as an encumbrance (gravamen) on the property in question and “cloud on title” rendered as gravamen sobre el inmueble. But if the reference is actually to a defect in a registered title, this is known (at least in Spain) as inexactitud del Registro, and perhaps in this case “cloud on title” could be rendered as inexactitud registral.
Note: I’ve used plenty of wishy-washy “mays,” “perhaps” and “maybes” above, because “cloud on title” is very close to being one of the true intraducibles of Legal English.