Quoting the definition of Riskware by
Malwarebytes:
"Malwarebytes uses the detection name “Riskware.” to apply to a category of programs or applications that put the user at risk in some way...
Programs might be termed as “riskware” because they:
Violate the terms of service (ToS) of other software or a user platform.
Block another application or software from being updated and patched.
Might be illegal to use in the users’ country.
Could be used as a backdoor for other malware.
Are indicative of the presence of other malware.
"
I trust Malwarebytes but not Avast. In your place I would delete the app even more if it hasn't been used in months. As a general rule, I eliminate everything that is not essential for my daily use of the phone.