I'm no expert but if I remember correctly a low hemoglobin indicates bleeding. Now the cause for this could be a variety of issues related to IBD or other things. For example in women this could be common due to menstrual cycle. I personally have had a slightly low hemoglobin which my GI told me I had mild anemia. I was told to eat foods rich in iron such as oatmeal, cereals fortified with iron, meat (red, turkey, chicken), shellfish, legumes, eggs, spinach, etc. To absorb iron better, it is recommended that you take it with foods that contain vitamin C such as citrus and berries. I was also told to cut back on caffeine or not consume with meals as it makes iron harder to absorb.
Of course everyone is different so you may have to adjust this based on the foods you are able to tolerate. I personally work with a registered dietitian to help me navigate this. She has been able to take the information my GI provides and made a customized diet for me. On a side note, my UC is not currently in remission but I am pretty much symptom free and on my way to remission. I have also found I have no food intolerances other than slowly adding back fiber into my diet now that my fecal calprotectin is dropping (currently at 159).
I hope this information helps!