I do not have precise, practical advice for how you should grade, but my comment will hopefully be of some general value.
What, specifically, do you hope your students will develop over the course of your class? My advice would be to keep this closely in mind and make that the focus of your grading. Ask: to what extent do the papers demonstrate the essential aspect of our class?
It will, of course, be helpful to the students to know what you're focusing your attention on when you are grading. So, let them know what you will be paying attention to, and give them a rubric if you can. That way they can put their effort into showcasing how they have coherently learned the subject matter.
The overlap of your content and ELA, of course, is in comprehensibility and coherence. It is possible for the paper to fail to communicate; if you are unable to decipher points of the paper, then they have not merely failed in an ELA sense. So, technically, grading them on their ability to make a coherent argument and express it clearly still falls well within your class. They can only succeed if they were able to express their ideas effectively, even if they break some ELA rules along the way.
However, this doesn't prevent you from marking up any ELA problems you happen to notice as you are grading. While it may not be part of their grade consideration, and it need not be something you spend additional time on, you can probably provide some useful ELA feedback to them. You may note that there are grammar problems, that spelling is an issue, that there is a problem with the coherence from paragraph to paragraph, etc. These comments can be useful to students even if a grade is not dependent on them.