my sea star does as well, it is a fill and vent plug, in hot weather the excess will seep out slowly
very important..........very important
ensure you keep your steering topped (check the levels) up as some folks have been killed on ribs and boats where they have had a leak or loss of fluid and then they lose the steering at some speed. so keep an eye on it, i am sure it would not vent enough out to cause a problem but always best to be safe than sorry
for filling up get a pippette from a beauty shop or similar and it is easy peasy then