where does it say its open source? I do not see this anywhere. what is the stated license?
assuming it does say this somewhere, have you attempted to contact the developer to request the source code? for example here https://app.macoou.com/inquiry What was the result?

if yes to the above and no resolution:

could try reporting via whatever google's mechanism is; "flag as inappropriate" i guess
could contact the SFC https://sfconservancy.org/copyleft-compliance/ they are the only org I am aware of that does this kind of thing as a general activity; I doubt they would be interested in this little hobby project-looking dev
if the dev is using FLOSS code, for example which was published under GPL, and they are not complying with the license in redistribution, then you could notify the devs of the GPL code
if you wish to pursue the matter independently you will need to find about about the dev's local jurisdiction and how to carry out a legal action there. looks like that would be japan.