I noticed this morning some kind of fungus growing in the soil of my Rubber plant. Never seen anything like this before. Is it an issue?
Growing media (potting mix) is made from a range of natural ingredients including peat, coir fibre, composted pine bark, etc, all of which break down over time due to the action of soil microbes and microflora, including various fungi. When potting mix is warm and consistently moist, as it usually is for indoor plants, then occasionally the fungi on-board may develop mycelium (threads) and fruiting bodies (mushrooms or toadstools).
These fungi are not generally harmful to plants when only one or two mushrooms are visible but if the entire surface becomes involved, then some damage may occur to your plant because they are an indication that the mix is overly moist and maybe the plant is in a position where air circulation around it is minimal.
Move the plant to a spot where it receives better ambient (not direct sun) light and good air circulation and cut back on watering to allow the top 5ccm or so of mix to dry out before more water is added. Your mushroom crop should disappear relatively quickly!