What could it be when under my tounge looks white? i do have low iron. is this something serious?

Don't delay: The ventral surface of the tongue is a place where a variety of things occur. Anything from as simple as a variation of normal, to a primary place for oral cancer. Lots to look at here. Consider risk factors include smoking. Underneath the tongue is an area where something should be examined without delay. It may be nothing. But it may be something.

White: "white" should always be evaluated, and the sooner the better. See your dentist and ask if a referral to an oral surgeon is in order.

White: Can be leukoplakia which is precancerous. May be candida infection, but needs evaluation.