Once you get the soil dry enough that watering will do no harm, consider watering for a week or two with dilute hydrogen peroxide, "just because". It might knock down the algae a little, and it might discourage any mold who are thinking they feel right at home in that wet, under-aerated soil.
BTW, if you have soil that is much too water-retentive, and don't want repot (*) again right away, you can remove some of the excess water with a wick, if the soil in the pot presses against the holes in the bottom of the pot.
Find a water-wicking fabric that is thick and fuzzy, like cotton flannel or cotton toweling.
If the soil-less mix doesn't touch the towel, put a cotton ball or two into some of the pot's holes, to make wicking contact between the soil in the pot and the flannel outside.
Set the pot on top of the flannel so that there is a continuous path from the soil, through a cotton ball if necessary, to the fabric outside the pot. Capillary action will assure that the wick is as wet as the soil in the bottom of the pot, right next to the holes.
Maybe you can just splay the wicking fabric around the saucer the plant sits in, and rely on evaporation to pull excess water out of the pot.
Maybe you have to set the pot somewhere the wick can
dangle down into some other saucer or pot, and wick water out of the pot by capillary action PLUS gravity. This "dangling wick" makes the soil column act as if the pot were tall enough to stretch from the lowest edge of the dangling wick to the soil surface. Any "perched water" remaining will be in the towel, hanging below the pot, not in the pot drowning the roots.
With an effective dangling wick, you can get away with a too-water-retentive soil mix, OR over-watering. I don't think anything can save a plant if it has BOTH water-retentive soil AND over-watering!
When you pull the excess water out, air will rush in to fill the voids in the soil. Once they are air-filled instead of water-filled, gas exchange will speed up 10,000-fold and the roots will be that much happier. (**)
{Edited to change "100-fold" to "10,000-fold" after I looked it up in Wikipedia.)
http://garden.org/ideas/view/R...
(*)
Re-potting includes knocking most of the old, soggy soil off the roots and replacing it with a different, faster-draining mix. If you have much root-rot going on, that soil will fall off easily.
"Potting up" is just moving a root ball to a bigger pot.
(**)
"Typically, a compound's diffusion coefficient is ~10,000× as great in air as in water."
https://en.wikipedia.org/wiki/...
and:
"
Effective diffusivity in porous media:
The effective diffusion coefficient describes diffusion through the pore space of porous media.[6] It is macroscopic in nature, because it is not individual pores but the entire pore space that needs to be considered. The effective diffusion coefficient for transport through the pores, De, is estimated as follows:
Effective diffusivity De = ( D x εt x δ ) / τ
where
D is the diffusion coefficient in gas or liquid filling the pores (m2s−1)
εt is the porosity available for the transport (dimensionless)
δ is the constrictivity (dimensionless)
τ is the tortuosity (dimensionless)
(I love the variable names "tortuosity " and "constrictivity".)
"The transport-available porosity equals the total porosity less the pores which, due to their size, are not accessible to the diffusing particles, and less dead-end and blind pores (i.e., pores without being connected to the rest of the pore system).
The constrictivity describes the slowing down of diffusion by increasing the viscosity in narrow pores as a result of greater proximity to the average pore wall. It is a function of pore diameter and the size of the diffusing particles."
