Methylated spirits on a cotton bud very gently is fine, but it is speculative given the limited description of what you have. Maybe talk to a Conservator at a Gallery or Museum for specialist advice or even ask them if they could do it for you?

A lot of my 1970s and 80s Kodachrome trannies have been cleaned carefully with meth., so too, any Fuji reversal stock—just dust and specks, never anything dreadful. I have heard Ilford has a proprietary film cleaner that is highly recommended by the name is obscure. A friend once dipped 4 of his filthy (Ektachrome) slides in a cup of warm soapy water. I grieve to relate...

If the grime is really stubborn, I suspect there might be some damage to the slide during cleaning through a combination of age and exposure.