Didn't understand relationship in FROM part of query call

The syntax for a child relationship is:

[namespace__]relationship_name__r

So if the relationship_name is Impacted_Products and the namespace is MC, your sub-query would be:

(SELECT Id FROM MC__Impacted_Products__r)

It looks like you used [namespace.] instead of [namespace__].

Note that you can always run this script to get the verbatim value you need to use in place of MC__Impacted_Products__r above:

for (ChildRelationship relation : SObjectType.Parent__c.getChildRelationships())
    if (relation.getChildSObject() == Child__c.sObjectType)
        system.debug(relation.getRelationshipName());

Where searching for the Child Relationship, you must use the Plural name of the Child Relationship object. For instance, if searching for the child Contacts on an Account it would be:

SELECT Id, ( SELECT Id FROM Contacts ) FROM Account;

In your specific case, try cyxs__r (I am assuming the plural of the cyx object is cyxs).