You can "decrease" the mass. You can only displace it. So, your description indicates and observation where the most obvious (original) material location lost some mass.
A single reactant means that none of the combination, oxidation (by other compounds) or substitution reactions can apply. The reduction in mass from a single reactant is a decomposition, like heating calcium carbonate (limestone) to form CaO (lime) for cement production, with the loss of a
The most likely type of reaction is a Oxidation or Combustion reaction. Oxidation is most commonly a type of decomposition This assumes that the Oxygen as a gas is not recognized as a reactant.
An example would be the burning or oxidation of octane ( gasoline )
In the combustion of many organic materials the compound breaks apart and forms Carbon Dioxide and Water. The formation and escape of these gases creates an appearance of the loss of mass.
If all these gases could be collected the the mass of the products would equal the mass of the reactants.
For over 100 years the prevailing scientific theory was the theory of phlogiston. This theory was the idea that there was an unseen material in matter called phlogiston that was changed into energy when something burned. The loss of phlogiston accounted for the observed loss of mass. Even when active metals like Magnesium were discovered and burned resulting in an increase in mass, the empirical experimental results were discounted because the evidence did not correspond to the accepted scientific theories of the time.
Lavoisier's theory of oxidation finally replaced the theory of phlogiston as the explanation of the apparent loss of mass.