Hello Welcome in H.C.M. Regarding your query for a white patch on the under surface of tongue, which is neither sore nor congested, appears to be Leukoplakia. No doubt leukoplakia is a precancerous change in mucous membrane. Firstly I would like to suggest you to get your self examined by ENT doctor as to rule out any adjacent sharp object which might be rubbing it like tar of teeth or sharp edjes of teeth. If find so that should be corrected and watch for reversal of mucosa to normalacy. If still it persists than excisional biopsy is the only investigation which will disclose the fact. If histology report is non malignant than excisional biopsy should be considered as curative step on the other hand if histology report comes as malignant than final onchological consultation become necessary. I think above answers your query. Regards.